This is a known issue and Microsoft says it is intended behavior: https://docs.microsoft.com/en-US/outlook/troubleshoot/user-interface/smtp-address-in-users-profile-not-change
The only fix is to recreate the Outlook profile (or add a new Outlook profile).
Warning: Many things can be linked to an Outlook profile, including PST files, additional mailboxes, and additional calendars. When a new profile is created, these items must be reconnected.
For steps, see: Outlook - Add or Remove an Outlook Profile
For assistance, contact the Service Desk.