I play on the web. I tend to vary my strategy based on the level one deck, but I always buy the vamp and jack when they come up. The minion was stuck in place on the path with red spurting off of it - like the hit animation - but the defenders did not continue to attack and the heart that shows its health was completely black. I know that it happened on different floors each time but it was always below the fifth floor. I do usually put on the fast forward. I don't remember if there was another minion that was on the screen at the time.
I'll try to keep a better record for you if it comes up again.