I've noticed that if I give an NPC multiple levels, it sometimes results in HP bugs. Screenshot attached below. The sixth level field was empty and I couldn't modify it until I went back a level and then forward again. It's a little weird. Not sure what's causing it, but if I hadn't hit the drop-down for the HP calculator, I'd have missed it and it would have affected a character without my knowing about it, so... thought I'd mention it here.