It took some time for this big to surface. I traveled far across! World and encountered many mobs. Then, mobs started disappearing from view. I fixed this one my slaughtering my many farm jobs and many wild jobs. But after traveling more and more it's now very bad. I was in a boat that disappeared underneath me and I started drowning. My boat (and horse in it) were gone. Then I swam away and they reappeared. I got lost and they disappeared. I swam to a local island and made a new boat. Once I placed it in the water it disappeared. So did a second boat. I can back up and see the boats. But I go close and they disappear. I'm stuck in the middle of the ocean. At this point this is 100% reproducible for me, after exiting and saving more than a dozen times.
They are not just 'not visible', I can't hit or use them. They are actually gone.
More, I also have a parrot with me. It also disappears and reappears