When repairing an item with mending, even with no other mending items in inventory and just holding the mending item, majority of the XP goes into your XP bar and not towards repairing the tool. Noticed it on a diamond and then on a netherite pickaxe but appears to affect any item with mending. I would say only about 20% of the XP actually goes into the items durability bar while the other 80% goes into your XP bar. Makes using mending very frustrating and repairing anything takes absolutely forever. Just to be clear after an equipping all of my mending armor and removing it, along with any other mending tools from my inventory, and just holding a mending pickax. Most of the XP was going into my bar