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.
