What I expected to happen was...:
Mobs captured in minecarts/boats to remain in those minecarts/boats unless they are broken.

What actually happened was...:
Mobs are no longer in their minecarts/boats after exiting and reentering the game. Specifically, I had a villager in a boat in my base that upon reentering was no longer in the boat. Also, I have a ghast blaster with nine ghasts that are no longer in their minecarts after exiting and reentering the game. I noticed unloading and reloading the chunks had no effect, neither did going back and forth to the nether. It happens reliably though when I exit and reenter the game entirely. It doesn't matter if the chunks are loaded or unloaded when exiting and reentering, it happens each time.

Steps to Reproduce:
1. Name a mob.
2. Put in a minecart or a boat. I put the villagers in boats and the ghasts in minecarts.
3. Save and exit the game.
4. Reenter the game. The mobs will not be in their boat/minecart.
5. Optional: I started having more issues with the ghasts after having snow golems throw snowballs at them. Also, all nine ghasts are named the same thing, "Ghastly Boi". The villager was named "Billy".

Additional info: v1.17.30