We've figured out what's happening here!
So to add a void into a slab with no footing, you're going to want to tick the "Thickness Only" tickbox in the slab tool.
There seems to be a bit of an issue in the tool where you aren't able to edit an existing slab to become "thickness only" which we'll have to fix, so you'll have to redraw the slab which is easy enough to do, but still not right.
One other problem with the tool is that you always need a specified dimension in the Footing Width and Footing Depth fields in the slab tool otherwise the thickness only option won't work. We'll also have to fix this.
Once the thickness only box is ticked it won't matter, so even if you just add 1mm depth and width and then tick the thickness only checkbox it will work.
Thanks for picking up on this issue! If there are any further Items you wish to add just let me know.