Steps to reproduce:

1. Place down 2 rails of any type connecting to each other
2. Place a boat one block away on the same line as the rails are on
3. Place a minecart on the rails
4. Push the minecart into the boat. Make sure to have enough velocity when pushing the minecart.
5. If the boat is now considered in the minecart, continue. If not, repeat steps 1-5 until so.
6. Place any mob that would go into the boat
7. Right click on the boat to enter it
8. Leave the boat. 

Observed Result:

The boat/minecart glides off similar MC-14. I was able to reproduce all of this. However, when the mob is killed, the boat/minecart loses its momentum. However, MC-277298 shows that the boat/minecart could not be moved otherwise.

This occurs in both the vanilla version of the game and with the minecart improvement datapack.

Difference with MC-14:

The difference is that MC-14 is specifically about stacking minecarts, but this issue is with a mob in a boat in a minecart and that theoretically, it wouldn't be able to retain any velocity.