Got the same bug, but with a leashed donkey. First it got invisible, and the leash disappeared too. Then, I continued to play normally and a zombie killed me near the invisible donkey, and it crashes my game whenever I enter the game. Will try to use a editor to remove it. By the way, it's on 19w08b.
@Matthew James Riley The problem is, that if the item has too many enchantments, I can't apply the Mending when I get the book.
I can first apply the mending one first if I got my hands on one, but it's not so easy even when AFK fishing.
***Edit: Fire protection can be applied mutually to the others too!
I don't know about you, but for me a chunk keep having full sky light, and some chunks keep reseting to light level zero, even with torches and lava. I'm no expert, but may this all be related with a light miscalculation based of the chunk coordinate?