There may come a time when you see Unknown User listed in your Teams chat history instead of the name you expected to see. All the chat history is still there, you can communicate with them still and all features seem to work.
I preface this with, security remediation changes are a great thing to keep you and your company safe. Some changes can however have unintentional (at the time) consequences that only rear their head later.
There were some security mitigation recommendations put in-place to limit user account visibility to non-admin users. One particular remediation had an unintentional consequence of blocking regular users from being able to read user account information in all O365 apps.
Putting the one variable back to default and having the users logoff/logon resolved this issue.
Open an elevated PowerShell command prompt and run the following:
Connect-MsolService Get-MsolCompanyInformation | Select UsersPermissionToReadOtherUsersEnabled
If it returns False, this will be the issue. It will need to be changed to True to resolve the issue.
Set-MsolCompanySettings -UsersPermissionToReadOtherUsersEnabled $True
Now when you run the following, it should return True. Users will need to close and reopen Microsoft Teams (and any other affected applications) to see the change.
Get-MsolCompanyInformation | Select UsersPermissionToReadOtherUsersEnabled