This is most likely to do with accuracy settings. I've seen it as well.
The simulation checks for collisions at 33% accuracy and that's how it works out where collisions are in the bar at the bottom and when the tool turns red. If you run simulation at a higher accuracy like you are (66%) then it realises it isn't a collision and doesn't stop.
Not very helpful but that is what I believe is happening... Hopefully it'll get fixed one day.
