Exchange Admin Center (EAC) automatically overwrites the regional user principal name (UPN) and replaces it with UPN from UPN suffix list in Microsoft Exchange Server 2019 or Exchange Server 2016. This issue happens only in cases where the user account is created with the UPN that isn't present in UPN suffix list. This issue occurs after doing changes to any user's mailbox property. As a result, users can't sign in to various resources by using their regional UPN.
To fix this issue, install one of the following updates:
Learn about the terminology that Microsoft uses to describe software updates.