I feel it's due to the users being created before the sort field was created. All the users created after the sort field was created are sorted correctly. The old users sort fine when opened and saved which I need to do anyway.
I'll mark this solved but wanted to add this for others to see if they come across this issue. I can't expect the system to sort by fields not yet available to a user I guess. That's how I am going to read this.
