There's a bug with + HP items, which don't actually add health to your character, but only raise the potential maximum health (while awarding no actual health). However, that's not the bug. The bug is how these items interact with the process of loading games.
Standard RPG behavior. If it would allow +HP equipping and unequipping them gives free health. That's why such systems only raise max HP. However if fully healed, and after loading your HP is lowered by this "+HP" then yes, that sounds like it's an unintentional bug.
Actually, it doesn't have to be implemented in a way that it can be used as a free healing device. If you have 10/60 HP and you equip an item that gives +20 HP, you should go up to 30/60 HP. If you then take damage and are down to 15/30 HP and remove item sporting the +20 HP bonus, your health goes to -5 and you die. That's because the HP boosts items are always tacked on to the end of the characters health pool, so they're the last to be chewed through when damage occurs.
I've seen games that do exactly that with + HP items and temporary health boost spells that specifically warn you of that danger. It's more of a "phantom health" design.
