I use version Immersive + Vehicles-1.12.2-20.1.0, the problem with the disappearance of vehicles was solved, but not 100%. They still disappear on the server, and here's how: when a player drives a car and abruptly leaves the game, it disappears, even if the player stopped on it, but immediately left the game, it disappears. And here's the question: where do they go to? is it worth worrying about the fact that they actually stand somewhere?

Is this ONLY when they leave while they are in the car?

Can confirm that

If you disconnect while sitting in a car, it disappears.
Sometimes it respawns on rejoin sometimes not

Fix idea: Dismount on disconnect

Well i don't know exactly how to do it, but google (Yeah i know google lies sometimes...) says: Get on disconnect the ridden entity (Car) and then "Eject" the Rider -> Here

Yeah, that's likely Minecraft entity syncing. I don't know I can dismount on disconnect, as I don't nessicarally get a disconnect request if they crash or their computer freezes. What I CAN do is make a fake entity that they ride that's not the car so when they disconnect they just make that go away rather than the car itself. Will also work nicely with sit-able decor blocks. This will take a bit to implement, but is a better way to handle entity riding and may allow for complete entity abstraction in the future.

Fixed in V21.2.0.