I have an enchantment mod that expands enchanting. One of the enchantments is vitality which increases the hearts of the wearer; however if that piece of armor is removed, the hearts do not adjust. This causes the game to think you have negative hearts and therefore if you take any damage at all, you get killed.

This shouldn't be caused by HealthOverlay. It's a client sided mod. It can't modify your "real" health. You should go and report it to that mod's issue repo.

Except without your mod, the hearts are added and removed as expected. How do you explain that?

Edit

After some more testing, it appears the mod that was killing me was not this mod; however there is still the issue of the hearts not being adjusted when the armor piece is taken off. So until I get hit by a mob, the hearts appear as though the armor piece is still equipped.

If this is still an issue with the mod adding the enchant, I will happily take up the issue with them.

Looks like a client side sync issue on their end. They're probably changing the MAX_HEALTH attribute on server side only without doing it on both sides or syncing it. So the sync currently happens when the health gets changed via damage or something else and not when the armour piece is taken off.

Thanks, I'll take up the issue with the mod author.