To avoid this problem (walls and ceilings erasing), just use the room tool to build your platforms inside houses instead of the new platform tool. The room tool has the new up/down arrows for platforms so you can use that instead. Build your "room" and use the new up/down arrows to raise or lower a platform, and then delete any walls you don't want around the platform.
The reason why the platform tool is erasing walls and ceilings/roofs is because the game is reading the platform as a new room... and a platform has no walls or ceilings, thus this new "room" won't either. But if you use the room tool instead, any existing walls/ceilings will stay put.
In a nutshell: Use the platform tool outside, use the room tool inside.