I don't know if this has been reported. I searched for the issue for fifteen minutes and it isn't here...

Whatever. I've noticed in your new update that minecarts containing players will freeze after about ten minutes of afking. It broke my mob farm which uses minecarts to improve efficiency. Anyways, I also noticed this happening with the collection hopper minecart, which would not only freeze but fly off the track and float in midair. To fix it I had to use /gamemode c and break both minecarts and replace them. When broken, the minecarts wouldn't actually drop themselves.

I have noticed this issue with sheep, and also thrown trident entities, arrow entities, and most mobs that spawn at night, like zombies, skeletons, creepers, and spiders.

If this helps, I think the issue has something to do with the new bandwidth optimization stuff, but I'm not sure.

Unfortunately I could not include a recording of this happening as I have never actually seen it occur. All I have seen is the frozen entities when I come back from afking. 

To recreate this, make a ten or fifteen block long minecart track. Put two powered rails at each end, and blocks behind them to create a bouncing effect. Place a minecart on the track and get in it. Move forward manually until the powered rails kick in, causing you to move back and forth on the track. If it helps mine was at y = 180. Go afk for ten to twenty minutes, and when you come back, the minecart will be frozen in place.