It probably has to do with the numbers you are using.
Read up on levels here: https://sweethome3d.com/userGuide.jsp#addingLevels
and here for levels below 0: https://www.sweethome3d.com/blog/2022/0 ... esign.html

Floor thickness is exactly what it says: the thickness of the floor. The lowest level has a floor thickness of 0. You can see that because the setting in the modify level dialog is grayed out.