Seems fixed in 1.16.0 - but now launching directly off of a Slime Block behaves incorrectly.
I think it's just that it's using the same split-screen type as four-player, and just not displaying anything in the bottom right, because it's easier to program. Why they couldn't have put a map there is another question for another time.
It's not actually dependent on what world you're in, it's just dependent on you running up to the piston as it extends. I'd still classify this as a bug, but it is by no means exclusive to the Mario mash-up.
Affects 1.14.60 on Windows 10
Happens on Windows 10 and Switch as well, so it's not an issue with the Xbox One version specifically.