Hi @Steve Ashman , my colleague Bob did some digging and found that this is a known product gap, and unfortunately has been for a while. I've reopened the issue here and will reach out to the appropriate channels to bring it to their attention.
"Even though you have migrated the user from AD to Azure AD, the onprem SamAccountName is still intact on the user object, among other on-prem AD attributes. As a result, Azure AD picks those details and shows domain/user instead of AzureAD/user. This attribute cannot be modified or cleared through Graph APIs at this point, so there's no way to change the behavior"
I suggest submitting a new UserVoice feature request here so we can track the issue better. I'm really sorry about this! If I find a solution or learn of any updates I will post back here and let you know.
If you wouldn't mind, please mark this answer as "Verified" so other users may reference it.
Thank you,
James