I've seen several instances of this, not sure why

I've never seen this before to be honest. I've gotten them to negative health, but that was with devmode fuckery, and it was still a number.

Unless you can tell me how to reproduce the bug, I won't be able to do much about it as I rarely mess with damage systems during dev, so it happening to me is unlikely.

Just get seated in a vehicle. they will be NaN by default. I'm not sure if its save based or what it is

Closing as issuer says they haven't had this in a long time, and I suspect it was caused by importing 0 max health and / by 0, which is NaN.