In my base I have an area where I've healed some villagers and formed a village. Occasionally after a while of playing on my world, the villagers turn invisible, leaving only their shadows (see screenshot).

Steps:
1. Have a bunch of villagers (not sure if they have to be in a valid village or not).
2. Play for a while.
3. Villagers become invisible.

Closing the app seems to fix it, and I haven't tested if simply exiting the world fixes it as well. Also, I'm not sure if this affects other mobs.