What I expected to happen was...:
When any mob entity enters a mine cart or the player enters a boat, exiting the world/server should keep those entities inside their vehicles.
What actually happened was...:
It seems whenever the chunks are unloaded and reloaded where minecarts with entities are, the entities are ejected from the minecarts. This applies to players in boats when the world is exited and re-entered. Entities once in vehicles are put in the same location they were before, just not considered "riding" the vehicle.
Steps to Reproduce:
1. Push a minecart into a mob entity to have them enter the minecart. (Or just enter a boat)
2. Either exit the world and re-enter it or go far enough away for the chunks to unload, then re-enter the world/area. The idea is to unload the chunks.
3. Entities will be in the same position as the vehicle, but are not inside the vehicle.