Filtering issues by "Assignee is not unassigned" not working (BB-9937)

Issue #8779 resolved
Florian Metzner
created an issue!
(pay attention to exclamation mark after equal sign)
shows all issues, as if not filtered at all. The applied filter is (correctly) stated as

assignee: someone

The issue has already been addressed in #3367, which has been (falsely) marked as dupplicate of #502 ("assignee: no one" didn't work at that time).