Well these 1 and 2 are pretty weird. No idea why a new trapped one would appear. There is only supposed to be two of them (half of starting five rounded down). And I have no idea what killed you there. The heretic would not spawn if the spot was somehow blocked and they only activate once you leave the space anyway. Boss did the stomp so somehow he thinks you're stuck.
The third case is pretty clear but luckily not very common one.