What kind of group is that? Have you checked the audit logs, they should give you an idea of what's happening: https://learn.microsoft.com/en-us/microsoft-365/compliance/search-the-audit-log-in-security-and-compliance?view=o365-worldwide
User repeatedly dropped from Group
Jonathan Green
1
Reputation point
I have a user that repeatedly gets dropped from a group. I add her back and 24-48hrs she's dropped again. Any thoughts?