A user keeps being removed from Office 365 Teams Group for no apparent reason. The group admin keeps re-adding the user, but within 12-48 hours the user disappears from the group members list again.
Azure Audit Logs show the user being removed but without any explanation of why this happened.
Activity Type: Remove member from group
Category Type: GroupMembership
Display Name: Microsoft Teams Services
After further investigation, it appeared that the user in question was listed in Office 365 under Shared Mailboxes, even though they had a full Office 365 license and could log in to Email, Teams, and other Microsoft services. The most likely reason is that the user had previously left the organisation. Their account was deleted, but then shortly after the user re-joined the company and their account was re-activated.
After converting the shared mailbox to a normal user mailbox the issue went away.
Microsoft Office 365