Does that work?
I'm thinking it has to do with how it stores and applies the user filters, probably based on a User ID or something.
I was just trying the custom route because I thought it might work, but it doesn't seem to work consistently if I try to populate the field by calculating directly from [User], which is a drag. Feeding [User] values through regex works in some contexts but not in others.
For example, sorting a library view by
regex([user], name)
will sort the library into a 0 and a 1 category, and correctly gives me everything tagged with "name" in the 1 category, and everything else in the 0 category, which was encouraging. But if I try to use that same regex string elsewhere it doesn't work. For example, if I create a custom field and assign it that regex string as a calculated value, no files receive a 1 in the custom field (i.e. no matches). Similarly, when creating a view, if I use the regex to filter what files to show as opposed to using it as a search term it likewise doesn't work.
I'll probably just have to make a custom field and manually populate the fields unless there's some workaround I'm not seeing.