So when you join a Server it takes a little while, right? When you hit the ''cancel'' button during that time and then join a singleplayer-world QUICKLY, you get in one of the Worlds, either Server or your own world, but with the inventory of the Server and you cant mine stuff and ist wired. Cant really descibre it. I have no mods installed, pure vanilla. Ill attach vid for you.
PS: Tell host to remove 10 mb max vid size
PS: Im in 1.14.4 not in 1.15.1, i dont have a 1.15.1 Server.
PS: I think bad ping to the Server is required
PS: Changes made during wired Phase are permanent
PS: Sometimes game will crash when leaving world, just not in attatched vid
PS: The singleplayer world i joined is an exact copy of the Server-world, a backup, not the same block for block but i guess it Maybe has some Server files to allow this bug?