A critical support notice from Microsoft on March 9, 2026 is urging IT administrators to immediately halt broad deployment of the Outlook shared calendar improvements, following the discovery of two workflow-breaking bugs that particularly affect delegates and users of Microsoft 365 Group calendars.

The recommendation comes nine months after the software giant began rolling out the long-awaited feature set, which promised to replace the legacy MAPI protocol with a modern REST-based sync engine for faster, more reliable calendar sharing. But the transition has been anything but smooth, leaving organizations scrambling for workarounds.

The Problem at a Glance

Microsoft’s Outlook team is actively investigating two “class” issues that can severely disrupt everyday scheduling workflows. The first bug prevents delegates from accessing meeting attachments on shared calendars, while the second causes recurring events within Microsoft 365 Group calendars to misbehave—either by duplicating, disappearing, or failing to sync across member mailboxes. The fixes are still under development, and no ETA has been provided.

“We recommend that most organizations do not enable the shared calendar improvements broadly until these issues are resolved,” the support note states. “If you have already turned on the feature for your users, consider disabling it for anyone who relies on delegates or Microsoft 365 Group recurring meetings.”

Background: What Are Shared Calendar Improvements?

For years, Outlook for Windows has relied on MAPI/HTTP to synchronize shared calendars—a protocol that often resulted in slow loading times, free/busy discrepancies, and calendar appointment corruption. Recognizing the pain points, Microsoft began developing a new REST-based synchronization stack under the name “Shared Calendar Improvements.”

First announced in 2023, the feature officially started rolling out to Targeted Release tenants in mid-2025. Admins can toggle it via File > Options > Calendar in Outlook or force-enable it using Group Policies and registry keys. Once active, shared calendars that support the REST API (including those in Exchange Online and Microsoft 365 Groups) should load faster and show more accurate availability. The improvements also pave the way for richer calendar interactions across Outlook endpoints.

However, the rollout has been staggered and cautious. Microsoft’s own documentation warns that the feature is still not available for on-premises mailbox scenarios, and some add-ins might not work. The latest advisory raises the stakes considerably.

The Two Critical Bugs

According to internal Microsoft communications and support documentation updated on March 9, 2026, the two class issues under investigation are:

Bug 1: Delegates Cannot Access Meeting Attachments

When a manager’s calendar is shared with a delegate who has Editor or Author permissions, meeting invitations containing file attachments can become inaccessible. The delegate sees the meeting in the shared calendar but is either unable to open the attachment, receives an error message (“Can’t open this item”), or finds that the attachment link is entirely missing. This bug appears regardless of whether the delegate is using full mailbox delegation or calendar-only sharing.

For organizations where executives rely heavily on assistants to manage schedules—a common practice in legal, financial, and corporate settings—this bug effectively breaks a fundamental workflow. One IT administrator on a Microsoft community forum described it as “a showstopper for any C-suite rollout.”

Bug 2: Recurring Meetings in Group Calendars Become Unstable

Microsoft 365 Group calendars, which every group member subscribes to automatically, are particularly popular for team projects and company-wide events. With the shared calendar improvements enabled, recurring meetings set up in a Group calendar may:
- Spawn duplicate copies that shift across time zones.
- Disappear from members’ individual calendars without warning.
- Fail to properly reflect changed start times or recurring patterns.

Several users report that after enabling the feature, they noticed mismatched meeting times between OWA and Outlook, or that meeting cancellations were not propagated to all members. The behavior appears erratic and does not follow a clear pattern, making troubleshooting difficult.

Microsoft has not disclosed full technical details, but early analysis suggests the REST sync mechanism is mishandling the complicated series-of-events data structure that underpins recurring appointments in Exchange Online.

Microsoft’s Advisory: Pilot Only

In light of the bugs, Microsoft’s latest guidance is unambiguous: do not enable the shared calendar improvements for your entire organization. Instead, limit the feature to a small, carefully selected pilot group that does not include users who:
- Use delegate access heavily.
- Rely on Microsoft 365 Group calendars with recurring meetings.
- Work in environments where legal or compliance requirements demand reliable attachment handling.

The pilot should be monitored closely, and if either of the two bugs surfaces, the feature should be turned off immediately for the affected users. Admins can disable the improvements by clearing the “Turn on shared calendar improvements” checkbox via Outlook client settings or by reverting the Group Policy/registry setting: HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Options\Calendar (value RestSharedCalendar set to 0).

Microsoft also advises that before re-enabling the feature on a wider scale, organizations should wait for an official fix announcement via the Microsoft 365 Message Center or the Microsoft 365 Roadmap (Feature ID: 88699).

Workarounds and Impact

While the permanent fix is in development, admins have several interim options:

  • Use Outlook on the Web (OWA): Delegate and Group calendar issues appear to be less prevalent in the web interface, so directing affected users to OWA for critical calendar tasks can help.
  • Fall back to MAPI-only sharing: If the feature is disabled, Outlook reverts to the legacy sync protocol. Although slower, it reliably handles attachments and recurring meetings for both delegate and Group scenarios.
  • Split the pilot: Some organizations are using Group Policy to enable the feature only for users who do not serve as delegates and do not subscribe to Group calendars. However, this requires detailed user profiling.
  • Reduce attachment reliance: Encourage meeting organizers to upload files to SharePoint or OneDrive and share links instead of attachments when delegates are involved.

For large enterprises, the impact has been significant. One healthcare IT director noted that their migration to shared calendar improvements had to be rolled back after just one week because doctors’ administrative assistants could not access patient-visit attachments on shared calendars. The rollback itself required manual registry changes on hundreds of workstations.

When Will a Fix Arrive?

Microsoft’s support note does not provide a timeline, simply stating that the team is “working to resolve the issues as quickly as possible.” Based on the complexity of recurring meeting synchronization—a notoriously difficult area in calendar systems—some industry observers expect a fix could take months rather than weeks. The next scheduled update for the feature on the Microsoft 365 Roadmap is currently listed as “In development,” with no specified release quarter.

In the meantime, Microsoft is asking admins to send diagnostic logs through the Feedback Hub or the “Help > Contact Support” path in the Microsoft 365 Admin Center, to help identify edge cases.

Best Practices for IT Admins

Given the fluid situation, here are actionable steps IT administrators should take now:

  1. Audit your organization’s calendar usage. Identify how many users rely on delegate access (e.g., executive assistants, team leads) and how many active Microsoft 365 Group calendars include recurring meetings. Decide whether any broad deployment is worth the risk.
  2. Communicate with stakeholders. Inform department heads and executive assistants that the new calendar experience is temporarily on hold. Set expectations that a fix is coming but not yet available.
  3. Create a targeted pilot group. Select a small group of power users who rarely use calendar delegation or Group calendars—for example, individual contributors in IT or Engineering. Enable the feature for them and collect feedback on performance improvements, but warn them about the attachment and recurrence limitations.
  4. Monitor the Microsoft 365 Message Center. Any fix announcement will appear there first. Set up keyword alerts for “shared calendar improvements” and Feature ID 88699.
  5. Prepare a rollback plan. Document the steps to disable the feature en masse, whether via Group Policy or scripted registry changes. Ensure your help desk is ready to handle support tickets from users who find their calendars broken after a flippant enable/disable cycle.
  6. Test Group Policies carefully. The policy setting “Turn off shared calendar improvements” can be used to force a disable. Reversing it after a fix is applied is straightforward, but you need to test the re-enable behavior in a lab first.

Conclusion

Microsoft’s shared calendar improvements represent a significant architectural leap forward for Outlook, but the two unresolved bugs highlight how fragile real-world calendar workflows can be. The promise of faster sync and better reliability is tempting, but for now, the risks outweigh the rewards for most organizations.

By following Microsoft’s own cautious advice—limit the feature to a pilot, exclude delegate and Group calendar heavy users, and monitor closely—admins can stay prepared for the eventual fix without compromising daily operations. As one community moderator succinctly put it: “This is a case where the ‘wait and see’ approach is the only responsible option.”

We’ll continue to track this story and will report back when Microsoft releases a resolution.