Hello @Peter Bradshaw ,
Hope you had selected below option while joining device to Azure AD:
You can run dsregcmd.exe /status
which display current state of device as below, if status set to AzureADJoined=Yes
then every users part of Azure AD can login. Try using AzureAD\<UPN> format when you Sign-in to AzureAD joined device, if the issue sill persist then use Event Viewer to locate the log entries logged by Device registration service, Located under Application and Services Log > Microsoft > Windows > AAD
Hope this helps.
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