Still not 100% sure. The game crashed so I remade and couldn't replicate it. Weird bug I guess.

You are not the first that reported weird slow loss of hp overtime in d2r.

So there is something weird somewhere for sure.

I haven't had this happen yet but I was at a friend's house and he had the same bug. Just sitting in town and he was losing health very slowly over time. He took off all his gear and it went away, but we never were able to tell which item was causing the health drain. Nothing he had on would have been doing it intentionally