Ohh I got you now! Unfortunately you can't use that tiles to create an inner corner if the software doesn't intend them on different layers! Now we have 2 ways to fix it, tell me which one do you prefer:
1) I split the A5 file into 2 files so I can add more walls and ceilings options that exactly fit the tile size
2) I add the walls and corners options to a generic B-C-D-E file, so the software will intend them on a different layer