Seems to be working for me (if I am doing the same as you).
I cant tell where your reference line is for the perpendicular wall so I tried both sides.
I simply created all walls vertical and then just dragged the top of the wall on the right to slope it.
In Options > Preferences > Construction Elements - do you have legacy intersection on or off?
Do you have the latest hotfix for 17 (build 4005)?
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