Worn armor doesn't accurately show the durability that it has. When mending the armor (using the enchantment), the armor durability stays the same until it is picked up and put back on (see attached video). And while the video doesn't show it, the durability bar can be seen jumping around while taking repetitive hits (I couldn't recreate it like my discovery of it, so you'll just have to take my word for now). While holding armor in the player's hand while using mending, the problem doesn't occur, which of course means that tools aren't affected by this. I haven't tested it yet, but this could affect shields as well.

Producing the bug:
1. Put on some armor and lose some durability
2. Gain enough experience to mend the armor
3. Open the inventory, pick up the armor, and place it back on to see that the durability bar changed.

What should happen: The durability bar should accurately show the durability after mending worn armor specifically.

What happens: The durability bar doesn't update to show accurate durability, and the player must pick up the armor in the inventory and put it back down for it to update.