I saw this happening for the very 1st time yesterday in Jargon's Temple too. It happened after I changed my equipment so my 1st assumption is that it has to do something with added bonusses from special equipment (Rings, Amulets etc.) but I am not sure yet. Another explanation could be that some areas cause this strange behaviour to the Health points - and so far the Temple behind the Bandit camp was the only area where this happened to me as far as I can tell. When it happens, it is annoying for sure. Inside the temple is an area with chests on the side you can only loot if you reach the chests and get back to the water very quickly because otherwise you get your butt cheeks burnt.

I don't know how often I had to try and wait for that stupid Health bar to restore to have at least a small chance to survive the next attempt.
//Update: I just found out what caused it. It is not area related but in my case it is a ring which I have modified with an improvement + 2 to Life points and an amulet which offers + 2 to Health by default. I guess when you load a Savegame, the Game acts like if you wear the ring for the very 1st time and that's why the health bar starts restoring again each time after loading. Annoying for sure and I hope that it will not stay this way. What I am wondering now is that I am almost sure that I used bonus items for Health before in the Game already but without seeing the health bar restoring after reloading...odd. If you specialize yourself and want to use Bonus Items to increase your Health, you will be quite naked in combat if you have to reload lol. I guess I have to invest more points for Life in my Char build...Lord knows if this is a bug or a feature.