Fall through platform

So I was making this level where you start off on the inside at the bottom of a mug. You have to escape the mug by climbing the steps and dodging a moving platform that lowers into the mug (like a biscuit being dunked). Then you work your way around the outside of the mug and the finish it at the base, on the outside.

I made the interior for it in constructor and was just testing out to see if it all worked. It's not completely finished. But in MBG when I got onto the 4th step, I fell straight through it, and everything above that (apart from the walls of the mug).

Is there any way that this can be fixed (so I don't fall through)?

(dif v0 and map attached).

The best way I know to get rid of this issue is splitting the map into a few maps.
MBMap2Dif can be very capricious with round shapes, tubes, and such...