because there is no mechanic to "reset" a corp or keep track of corps that are leaving a red alliance to go unkos. we don't generally set every corp in an alliance kos or keep track of an alliances corp recruiting. its too much work.
If a corp leaves a hostile alliance its considered "neutral" that doesn't mean freind or foe.
If a corp leaves a hostile alliance and pirates we file a kos request.
If a kos corp (already individually listed as kos) leaves a kos alliance, it is considered kos.