How can I get the White Box to work?

I’m messing around with Ahorn, and I find the SMB3 White Box that’s used to get the chapter 4 Crystal Heart. I place it down and put background walls in the same screen. The problem is, when I use the White Box to enter the background, I can’t walk on the background walls. I checked the original area where it’s used in chapter 4, and there are no triggers for it. What am I missing?

Quick Edit: Just in case, I used the same background walls used in chapter 4 and there was no difference.