I also observed this behavior. I've killed at least 8 people so far, and each time it seems that if the player logs out or respawns, their body (along with all loot) instantly disappears. This typically gives me less than 5 seconds to loot someone before everything they were carrying is gone.

I presumed that zombie corpses were despawning because they never have loot on them, but I'm not sure about that.

I've had this happen when rejoining the same server. It seems to be a low, somewhat random chance.