Hi Pbaravik,
According to the description, I’ve tried to reproduce to the issue in my test environment and failed to reproduce the issue. Here is a screenshot about how I created a test user in Active Directory Users and Computers on the domain controller:
After synchronization, the synchronized user in Office 365:
Theoretically, the synchronized user’s UPN only depends on the on-premise user’s UPN attribute.
Based on the current situation, please provide the following detailed information:
1. Please check the user’s UserPrincipalName attribute via ADSI Edit.
2. If the way that you used to create the affected user is different from mine, please provide the detailed information about how you create an on-premise user, such as some screenshots. As a tip, please click Use rich formatting> Insert image to upload the screenshots in the post manually.
3. Create another new test user in the local AD to see if the same issue occurs.
Thanks,
Claud