Apparently, 1.20.50.21 has a fix, so that mobs that enter boats as babies are ejected out if they are too big. However, instead of running a single check when the adult grows, it seems to eject all large mobs out of boats. If they were put in boats before that update, they should stay in the boat.

How to reproduce:

1. Downgrade Minecraft preview to any version before 1.20.50.21
2. Create a new world
3. Spawn a baby horse/donkey/mule, put it in a boat, then make it grow up
4. Update to 1.20.51
5. Re-enter the world.

Expected result:

The horse/donkey/mule remains in the boat, as they were already put there as adults before the update

Actual Result:

The horse/donkey/mule is ejected out of the boat, possibly ruining any effort players may have put in to get them like that in the first place.