Playing on 0.61 last night had a fight with another survivor and knocked him out - his body fell to the ground at my feet, then disappeared.
I was going to tie him up with rope, but I couldn't see his body

I could still get the "Tie Up" screen message, so the character was still there. They had just become invisible.

The ground was only very lightly grassy, just outside Solnichny

Before I could tie the survivor up, they respawned and I was able to loot their rags - but the body stayed invisible