Hello, we managed to isolate the cause and fix it a few days ago.
The former moderator was not a member of any flared group. What we had to do was add them to another group with a flair, then from their profile preferences ( /u/USERNAME/preferences/account), change the Flair to “none” and then we removed them from the group.
Seems like a general bug where if you are removed from a group with a flair equipped, the flair stays despite not being a member.