Run the following cmdlet might help to diagnose:
Get-AzureADDirectoryRoleMember -ObjectId (Get-AzureADDirectoryRole | where {$_.DisplayName -eq "User Administrator"}).ObjectId
Get-AzureADDirectoryRoleMember -ObjectId (Get-AzureADDirectoryRole | where {$_.DisplayName -eq "User Administrator"}).ObjectId | where {$_.UserPrincipalName -eq $null}
If the second cmdlet returns any record, the issue is caused by the missing UserPrincipalName attribute.
The workaround is:
- On the tenant you can see every new admin pool accounts created successfully, however when checking the Notes Migrator Admins Role group only 1 account had been added as a member. Manually add the other accounts and they are will be visible in the admin account pool utility.
- Check the AzureAD roles applied.