Hello, I tried to reproduce your problem..without success !
You can see the floor of the basement , and the floor(blue tiles) of the level 0 . I made a notch in the floor in order to be able to see the basement floor.
It seems that your wall is is not at the border of your room .. Did you check you don't have two rooms stacked on top of one another ?
Hope it helps .
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MacMini 3GHz Intel Core i5 6 cores 64Go macOS Big Sur 12.2.1
