Room doesn't register as a Room in Build Mode

I recently decided to remodel a mansion I built. I deleted all the interior walls so I can restructure the rooms. My issue came up after I put up new room (and thus wall) locations and tried to apply wallpaper and flooring. The wallpaper wouldn't apply to the entire room when pressing shift nor would the floors. The wallpaper even applied to parts of the house exterior meaning the build mechanics thinks the wall is entirely on its own... I tried deleting all the floors and then click on the yellow outline to build up the floor that way but the yellow outline to "build floor" doesn't even show. After some finagling, I noticed that the application of flooring and wallpaper seems to follow the old room placements! I tried deleting all the "rooms" back to just the exterior walls, saving the game, and restarting the game, but the issue is still present. How can I get the rooms I build to register as their new room locations????