https://phabricator.wikimedia.org/tag/collaboration-maps/ redirects to https://phabricator.wikimedia.org/tag// .
This is a real hashtag for a real milestone (https://phabricator.wikimedia.org/project/manage/3219/) (without a board so far, not sure if that matters).
Tags that simply do not exist behave differently (https://phabricator.wikimedia.org/tag/does-not-exist).
Upstream: https://we.phorge.it/T15547