I think I may have the solution even though I haven't been able to reproduce this anymore after the first round of fixes. I think it's somehow caused by timing issues and/or ordering of things when heretics move at the same time the cross is grabbed. But the heretics shouldn't even be allowed to move at that point since you move first, grab the cross, boss does it's thing as it's getting hurt and then when it would be the heretic's turn to move, there actually are no more tiles to move to. So I think making them just not even try to take their turn on that scenario would fix this issue.