This bug has started happening to me and the members of my realm since 1.17.40, (NOT the Beta, but the 'live' release) we have only encountered this using boats.
Sometimes it happens every time you exit a boat, other times it won't occur at all. Seems random.
One thing we have noticed is that if the boat disappears for one player, others can still see it. They can then break it and as soon as it has been broken the player that couldn't see it originally can then re-see it floating/sinking in the water and pick it back up again.
When a specific player cannot see the boat, they also cannot see the mob/other player riding in the boat. The entire entity & other entities within the boat completely disappear until another player that can still see the boat breaks it. Once the boat is broken everything that had vanished, reappears.
This is workable if you are playing with others. If you are on your own and this happens, then the boat is lost seemingly forever.
This has happened on my realm as well. Half of our villager trading hall disappeared overnight before 1.18 dropped and we lost count of the amount of animals that despawned over the last month or so.
We have users on OG Xbox One, Switch, Xbox Series S & X as well as PC so this isn't just down to specific users hardware when it is occuring on realms.