With the most recent update, mobs and players will randomly disappear. The attached videos shows both disappearing without a trace, however there have been other circumstances where a mob will suddenly appear to stop moving and yet attack you while invisible, as if its soul left its body and started attacking me. I do not have any videos of this, but if I can get one, I'll add it.

The specific incident I will reference is shown in the three videos below (they're seperate because that's a limitation of the Switch's recording system).

https://twitter.com/i/status/1332784915809767424
In the first one, my friend (C) and I are travelling to a village. I jump into the water with my horse to push it across, but as it lands in the water, it vanishes. C says she can still see it.

https://twitter.com/i/status/1332784915809767424
In the second video, I ask her to ride it to prove she can see it, and when she rides it, she also vanishes. I ask her to get off and when she does, she reappears.

https://twitter.com/i/status/1332784915809767424
In the final video, the horse reappears without C on top of it. You can tell it's being controlled by a player because it plays the right jumping animation and it's moving faster than a horse's idle speed. Though it's not shown in the video, when she gets off, she reappears and the horse stays visible.

I'm not sure how to replicate this; it seems to be random. The world size was under 10 MB with this incident, though other worlds where the "ghost mobs" occur have been over 100 MB. For reference, C plays on Windows 10 edition, and I've had "ghost mobs" occur by myself, with fellow Switch players, iOS, and C on Windows 10.