That would appear to be telling you that your wall isn’t flat, or you created the section at an angle.
That was my first thought, but I’ve recreated the section on different surfaces, including new ones I’ve added to guarantee they’re square.
I can do a section facing the other direction with no trouble, then reverse it, and the triangle reappears.
Can you attach a model that shows this.
Section 4 is the problem, I’ve tried reversing Section 5.
If you make a separation between the construction elements, walls, floors, grouping each one separately, you solve the problem in the section.
Thank you so much! It worked!
You also need to sort out your face orientation. All the blue faces are the back faces and shouldn’t be visible when viewed in Monochrome mode.
Good point - thanks.