Hello Jonas U.,
Thank you for posting in Microsoft Community forum.
1.What do you mean "The "Logon as another User" button is missing"? Do you mean "Switch User" button?
2.Based on "it's impossible to logon as a new user for some reason.", did you receive any error message?
3.Can you logon as a new user before the devices joined to Entra-ID/Azure-AD domain?
4.Based on "it's impossible to logon as a new user for some reason.", do you mean local user on machine or Azure AD domain user?
If you have any question or concern, please feel free to let us know.
Best Regards,
Daisy Zhou