This was discussed previously T84833: When you close an unassigned task as "Resolved" from the Comment action, Phabricator thinks you want to claim the task and I have memory this was changed so closing tasks when un-assigned didn't claim to the closer but Phabricator is doing it again.
Current configuration:
https://phabricator.wikimedia.org/config/edit/maniphest.statuses/
That is visible only to an Administrator, but probably as already mentioned by aklapper it should be possible to set "claim": false. At the moment it seems the auto-claim is disabled only for INVALID and DUPLICATE statuses.