ATTENTION: I’m currently testing this bug so there are possible incorrect informations
If you kill a copper golem while it’s holding an item, it will drop only the copper golem loot (1-3 copper ingots) and not the item it’s holding.
Instead, if the copper golem is holding an enchanted tool/weapon/armor piece it will drop that item.
Sometimes if you damage the golem while it’s holding a tool/weapon/armor it will put in the chest the tool/weapon/armor but damaged.
So, in the end, my theory is this: until the golem isn’t holding any enchanted item, upon damaging it, the golem transfers the damage to the item (that’s also why it don’t drop a normal item)
Pls, don’t mark this as “not enough informations“ because that’s a place where we can find the conditions where this bug occurs
P.S.: Srry 4 the possible wrong grammatic, I'm Italian 😅
This issue is being tracked at MCPE-223782.