Mario goes off the map at this point

[ctrlVnt]

At this point there is no limit and Mario goes over the wall and ends up outside the map.

Before:

After:

[IGN-Styly]

More evidence

[IGN-Styly]

Specifically in the grass part close to the stairs, thats where you clip.

[IGN-Styly]

there is colisions but mario is shifted under.