This issue can appear during the migration process and may remain until the migration is complete. If this issue still occurs and the domain has been fully migrated, please see the attached links.
After migration, messages that have been received in the mailbox display "FYDIBOHF23SPDLT" as the sender instead of their name or e-mail address.
Per the article below, the solution is to add the old X500 addresses as custom addresses to the user mailboxes. This is done by the Engineering team.