Users Reappearing in the App After they have been Removed

Got nowhere with support, so I'm putting this out to the community. Does anyone else have issues with Users that have been removed from an App showing back up in the App at random intervals?

This has been happening repeatedly for the last few months, and it's really getting annoying. I have a lot of List User fields, and utilize the Default User List, because I don't want to have to edit each of those fields every time someone comes or goes. It's not a matter of access, because when they leave they are put on the Denied list, but it does mean that when things are being assigned, those old users expand the selection list from 50 people to 200.

I know it will happen whenever I have to edit old records, and so if the removed user is on the record they will reappear in the App, but it also happens completely at random, and whenever there is a big Quick Base release. I have been unable to track down a specific cause for the completely random occurrences, though. I do know that instead of getting better, it's getting worse. I've already had to remove users twice this week alone.