Slabs don't have a visible top/bottom surface in the plan view.
You have two options.
Place the walls in a layer that gets turned off in the plan view but is on the sections/elvations and 3D views.
Or turn on the 'cover fill' in your slab settings (floor plan display section).
You will probably then have to use the 'Bring Forward' or 'Send Backward' display order controls to bring the slab forward or the wall back in the display order because the plan will not adjust the display order automatically based on element heights.
Barry.
One of the forum moderators.
Versions 6.5 to 27
i7-10700 @ 2.9Ghz, 32GB ram, GeForce RTX 2060 (6GB), Windows 10
Lenovo Thinkpad - i7-1270P 2.20 GHz, 32GB RAM, Nvidia T550, Windows 11