in my single player game, my Golem’s inventory had an invisible Dev. item called xx_Golem_Hammer_t1. that same item could then be placed into my inventory. had to remove it to allow my golem to use it’s two remaining inventory slots. the golem was still harvesting despite these invisible hammers in the inventory, causing material to seemingly be lost for no reason until the “hammers” were removed.
no idea why, but it was just an invisible item in the golem’s inventory. it was a pure stone golem, no coatings, no mismatched limbs or special head. due to the items having no icon or name until hovered over, it is hard to say when these items entered the golem’s inventory slots.