Correct Display Name but Incorrect Username when searching for users who have had a name change.

This issue is resolved. I had to open up the accounts and come through every single attribute looking for anything at all that referenced the old name. There were a couple of things that the top level admins said were legacy and were no longer used in our environment, but I removed them anyway. Due to how long it takes Teams to sync changes I did not follow best practice and turn them off one by one. Thanks for anyone who viewed this thread.