I have found several issues when using items that provide a max. HP bonus (effect opcode 18).
1. Items with different HP bonus modifiers don't properly stack. I've tested it with the Spear of Kuldahar (which provides a fixed 8 HP bonus) and the Pale Green Ioun Stone (which provides a 10% HP bonus). When equipping the spear first, the HP bonus of the spear will be overridden by the Ioun Stone HP bonus. You'll get the correctly stacked HP bonus after reloading the save however. Equipping the Ioun Stone first will give you the full stone bonus and the full spear bonus immediately.
2. Current HP issues when equipping items with percentage max. HP bonus. Equipping the Pale Green Ioun Stone gives you a 10% max. HP bonus. There appears to be an issue with the current HP value however. If you save the game with a fully healed character while the item is equipped, your current HP will be lower by one or more hit points when reloading the saved game.
I've attached a saved game to test the described issues.