Symptoms
When Microsoft Graph-based free/busy is enabled for cross-premises availability requests, an on-premises requester that queries an Exchange Online mailbox might receive incorrect busy-time intervals that are shifted by the requester's UTC offset. The shift is both timezone-dependent and DST-dependent.
This issue does not return an error message to indicate the incorrect free/busy information. Additionally, the Graph getSchedule response is returned as DetailedMerged instead of Detailed. When this issue occurs, some item-level properties are lost. This includes Item ID, IsMeeting, IsRecurring, and IsReminderSet.
Resolution
To fix this issue, install the following security update or later updates: