I have a user Elena that has multiple times been dropped out of all her Teams and private channels.
The latest incident was on March 16th. This time we were able to track the root cause to the application "Skype For Business PowerShell Server Application" having committed the change at 2:31 am that morning.
There was no support activity at that time nor do we recognize this to be due to any of our corporate maintenance operations, so i suspect that this could be some regular maintenance by Microsoft.
Here is the screenshot after we were able to track the root cause to application "Skype For Business Powershell Server Application"