Here’s something I learned from a support ticket. I thought I encountered a bug, but it’s just a quirk of how the “is not one of” filter works, at least for custom user roles.
What happens is that when you use the filter "Is NOT one", it will get all the users that currently have a custom role, meaning any users without any custom role will not be shown.
So that filter only works on users that have a custom role. Those with zero custom roles are not included.
That’s not what I assumed about the functionality, nor is it my preferred functionality, but I can see how/why it would work that way.
If every user has at least 1 custom role, you won’t have this problem. But if not, the utility of the “is not one of” filter comes with this restriction.