That is not a bug, that is simply how the disagreement system works on iNaturalist. See the first reply by bouteloua on the thread you linked.
the disagreeing Diptera ID, by backing it up all the way to Diptera, is also disagreeing its an acalyprate fly
rw0’s disagreement went all the way up to Diptera, and that ID will disagree with any subsequent more specific IDs until retracted or changed.