So first you're gonna want to go to the Back 4 Blood icon in your library, right click and hit "uninstall"
Now the next step involves going to the steam store and looking for the game again, this time getting a slightly different version that comes with none of the jank and significant improvements.
So open up the store and type in "Left 4 Dead 2" and hit search, it'll be the top result with a different name to usual but still recognisable as the same game
Now that you've found it just pay and download, go to the library and boot up the new version
And just like that you've got a glitchless version of the game with far better gameplay!

Hope this helped improve your Back 4 Blood experience!