Exchange online - shared personal calendar dissapear

Mailbox Full Access should let A access B’s calendar, but it does not guarantee that B’s calendar will automatically appear and stay in A’s OWA calendar list. The limited details and disappearing entry need to be checked separately.

1. In A’s OWA, click the profile picture and select Open another mailbox. Open B’s mailbox and its calendar. Check whether A can see the details of ordinary, non-private meetings. If access is denied, the mailbox permission needs checking first.
2. Have the Exchange admin verify A’s effective Full Access permission and inspect the permissions on B’s actual calendar folder. Also confirm that the manually added calendar belongs to the correct B account. Private appointments are not a reliable test because their details can remain restricted.
3. Have B send A a calendar-sharing invitation through Outlook or OWA, with Can view all details or Can edit, depending on what A needs. Remove the existing shared-calendar entry from A’s list without deleting B’s calendar, then accept the invitation and refresh OWA.

If the calendar works through Open another mailbox but disappears from A’s calendar list even after accepting the invitation, include that result in a Microsoft 365 support ticket. It narrows the problem to how the shared calendar is added or retained. Since OWA is affected too, clearing the desktop cache or rebuilding an Outlook profile is unlikely to address the whole issue.