Teleporting a vehicle and shortly adding passengers causes the vehicle to be left in a strange state and for players who are added as passengers to remain able to walk. To help demonstrate this, I wrote a small plugin. It can be used as follows:

1. Get in a vehicle. For my testing I got onto a saddled horse.
2. Now run /demo. Make sure you have about ten blocks of space in front of you, as it will teleport you about ten blocks forwards, and one block above ground.
3. Observe that the horse remains floating one block above the ground, while you are able to walk around.

This behaviour does not occur on 1.15.2. The source for this plugin is available here:
https://github.com/nicegamer7/PassengerBug

Additionally, I've included a /demo2 command. This command does not work on wither version, however, if it's meant not to work, it should probably be added to the Javadocs.