why does my floor's height increase when i place walls

This is my first time making a house from scratch so i started off with the walls and the platforms. I'm trying to build walls, but in certain areas my floors increase in levels. Some areas don't make the floor change when I build walls, but there are some areas that do. There are even some spots where I try to build walls and it hightlights some of the objects in the house yellow, but when I place the walls, those objects don't even get deleted so it trips me out. When I use the room tool, it's perfectly fine, but when I use the custom room tool or the wall tool, it increases the height of my floor and my objects get removed so I have to undo the walls. Is this a glitch or am I doing something wrong? I checked my second level and there was nothing up there that could've possibly affected my floors downstairs. The photo shows what happens whenever I try to connect the walls. If I make separate walls that aren't attached to each other then it's fine, but once it's connected like in the photo, it highlights the houses floor and the furniture.