I was playing Minecraft 1.12.2 OptiFine and I have a tamed donkey, when I brought him home and went to the mine to get the resources I mined, the donkey disappeared.
Fortunately the donkey reappeared when I went at my home and back there. The next day I was fighting some zombies and skeletons for some resources, when I built the fence for cows, an invisible zombie attacks me.
The fun thing is that I couldn’t fight him back because I think his hurtbox wasn’t loaded properly so I couldn’t hit him. In the end it killed a villager in my village and I after I slept, the zombie was gone.