I would try putting all of the foundation walls in their own layer.
Then you can display just that layer with a FPCP at what ever height you want.
Or you could forget the FPCP and set all the walls as 'Symbolic' if you want to see them all as cut walls in plan.
Or set them all as 'Outlines Only' if you want a top view.
FPCP really doesn't work over multiple levels.
The only way to get it to work is to set up multiple floor plan views all with different FPCP heights.
Then place the views on a layout and crop and reposition so they join to show the overall plan you want.
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