The instant death in that scenario is intended (kinda). If you play a tile and can't walk to it (no matter why really, if it's blocked on otherwise unreachable) the level ending will trigger and in case of boss levels, they'll do the stop and hurt you for each unfilled square.
But the heretic being off the tile is something I have no clue about (and shouldn't happen either). I guess it was on that other tile but somehow decided to move even though the boss destroyed the tile it was going to move to...