See PHI42. Currently, maniphest.search incorrectly applies this default (group by priority) to all queries via Conduit.
The correct behavior is to apply no grouping constraint.
I think this is also a reasonable general behavior, and the current code seems to date from D6960 in 2013 and didn't seem particularly carefully considered.
This is a minor compatibility break -- saved queries which are more than 4 years old might change their group behavior. I'll note this in the change logs but expect essentially no one to be affected.