Not a thorough analysis by any stretch of the imagination, but I have a working hypothesis about the armor bit.
It looks like 3d armor checks the reason for the HP change before deciding how to intervene. If reason.type is “drown” or reason.hunger exists, then it skips the rest of the callback.
hbhunger, on the other hand, calls player:set_hp without passing a reason. It’s not mandatory for the minetest engine, but if it doesn’t give a reason, then other mods (like 3d armor) can’t react appropriately.