I would be guessing that you have used the Connect > Trim Elements to Roof/Shell command.
This uses the roof trimming body for the cut.
Anything inside/outside (depends on your choice) of the trimming body will be cut away in 3D (no effect in plan).
As you have split the roof into single planes, it is trimming to the body of one of those planes and everything outside of the trimming body of that roof is being cut away.
Cancel the trim and do it the 'old fashioned' way with a Solid Element Operation.
The wall is the target, select both roofs as the operators and subtract with upwards extrusion.
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