It's hard to see in your picture what the problem really is, but generally, when the walls' reference lines don't overlap properly
in height at the intersection point, the walls won't intersect properly.
One workaround would be to place the prop wall on a separate layer and increase that layer's intersection number so they
don't intersect, and then do a SEO instead, and then as a last step patch up the plan if needed. In fact, any wall-shaped item (a slab, perhaps) would do. I said
workaround
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