Hi Seth,
Currently, breaking DirSync and removing the user in question is a valid solution for this situation, and it is also a troubleshooting step for the issue.
As you said "All of them changed in 365 to the correct address except this one", it proves that the DirSync relevant settings and configurations are correct. Therefore we do not suggest customers modify or change any settings in DirSync, to focus on this specific user account in question we suggest you remove this user account and re-sync it.
Thanks,
Bourne Zhang