Basements will not create irremovable flooring between levels.
Fixed in Patch 1.55.105
Edit: Upon further playing while still dealing with this issue, I have also discovered that activating the camera (in your inventory) to take a photo of the set, has a new issue that I can't say i've seen before, but not 100% sure the patch is to blame, but once in camera (photo) mode while in the basement lower levels...the entire world disappers. All the items I've placed still exist, but walls, ceilings, floors, and everything else goes invisible. Only placed items and natural terrain remains. So I'm basically taking a pic of under the map.
Of the 3 included pics, the one of my sim (in the red shirt) shows the room in the background with walls and floor. Then the two other pics are of the same room viewed through the camera. Weird textures on the floor, invisible walls. etc. I used to like to take pics of sims interacting on the set.
Seeing the same issue in my game. 🤨
Don't know if it will help to identify the cause of this issue, but I noticed that if you delete only a part of the basement floor (in this case I only removed the center portion), the dirt floor will glitch through the rest of the un-removed floor (kind of like how objects glitch when they're stacked using bb.moveobjects).
