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

https://bitbucket.org/site/master/issues?responsible=!\\
(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 BCLOUD-3367, which has been (falsely) marked as dupplicate of BCLOUD-502 ("assignee: no one" didn't work at that time).