Sad, sorry bout that! ðŸ˜
More info on how it happens please. Was it the first hit (not counting when opening the chest of course) or can happen at any point? Did you kill the heretic the same turn you move there or earlier? Did you happen to have sounds on, does it play the cross grab sound? How did you kill the heretic (and where on the map was it) Looks like you had blink too, any blinks or potion usage?
Might need to disable boss heretic spawns completely for now unless I find a solution to this.
edit: Also, do you remember if there was a gem on that cross spot tile?