You can mix enviroments without editing data.

Enter a title.

Exit that title.

Go into another title.

Go to local play, Single map, then pick a map.

EX:

Since I'm in canyon, I'll select a Stadium map, since I loaded both stadium and canyon (You should not see a question mark)



Now, I'm in a Stadium map with a canyon car.



Here's the reversed version.



Nadeo please fix this (or don't, I'm also OK with that)

was known since the release of stadium, so yeah, not new, and not fixed, and probably won't be ^^

You can search for tracks built with this intention, by selecting a standard title pack (TMValley, TMCanyon or TMStadium) and specifying an incorrect environment

ie.. filter for TMStadium titlepack with Valley & Canyon environment.