The rooms in the map are randomly fitted together. Ill look over the different segments and see if I can figure out what configuration you got that caused this softlock.
Actually, I'm playing the game again right now, and I just discovered you can jump down through certain platforms. When I'm on the surface, these platforms sometimes aren't visible, but I remember the location of these platforms and am able to jump through them.
I don't think my first playthrough of this game had a softlock, I just didn't know about some of the mechanics of the game yet.