Steps to Reproduce:

1. Make sure the world's simulation distance is higher than your render distance
2. Enter area with mobs
3. Leave area until they disappear from sight
4. Go back into area

Observed Results:
Mobs zip back into their proper place

Expected Results:
Mobs should pop in normally

Also i tried to reproduce it but i found a glitch that might be related. When night came two villagers entered their beds and levitated away until they disappeared. I then started to record and when I left and came back into the area they reapeared back in their original spot.
I sped up the video to fit the file size