Why is there snow in my sunken rooms?

Question

So I've been experimenting with sunken rooms (where the house in general is built on a foundation, but some rooms are built lower on ground level with modular stairs making them accessible from the rest of the house).

My house currently has 3 such rooms:

• Living room
• Part of the bedroom
• Study

Unfortunately, given that it appears that there's snow on the floor of 2 of these rooms (bedroom and living room), I'm guessing there's a problem with the ceiling.

Here's how I built the rooms:

1. Place foundation for the room
2. Build the walls
3. constrainfloorelevation false
4. Remove foundation
5. Place bottom section of wall
6. Auto-level walls

My roof is basically just a 2nd story that I've surrounded with one of the small walls (not the "waist height" ones).

I've taken some screenshots to help clarify what's going on.