Mobs randomly go invisible, it mostly happens if you unload the chunks (I used a name tag so, its not despawning.) but sometimes it just happens. I don't know why it happens so I ran an experiment. I used a name tag on a mob (a slime) and just when around the world just doing  random stuff (farming crops) then came back after about 5 - 10 minutes and it was invisible, you could still hear it but you couldn't see the hitbox when I used F3 + B. The only way to fix this was to reconnect to the world. The mobs I have "seen" that have gone invisible are slimes, zombies, drowned, and creepers (oh no). I have cats, chickens, and cows and they don't go invisible so I think it may just happen to hostile mobs.