Ok so we know about a few other issues that're currently being worked on such as the gittering issue that happens when you're a connected client but we made some major progress in getting the bug fixed with multiplayer, here's what was currently fixed.
-> Connected users can now jump properly and can thus make progress with their friends
-> Connected users can now flip properly and thus can be a bit more chaotic.
->Hosts can now see connected drivers flip and jump properly
->Connected users can now earn achievements properly..... I think? idk I'm just one man and I break a lit of things
->Stopped an issue where the final boss would materialise and send the player to another dimension
->Fixed the receipt value not showing the proper cost of your unhealthy pizza settings habits. Now we can accurately bill you!
Multiplayer jump and flip fix
Source: Steam Community (opens in a new tab)