Solids refuse to Connect or Connect to Nearest

Two linear solids (shape) - one say a wall, the other a sloping roof slab above it;

Why would the wall top refuse to Connect or Connect to Nearest with the roof/soffit?
In the case of Connect, the roof cuts back and forms the mitred edge but the wall flickers but doesn't extend up.
In earlier attempts, the wall end extended to meet imaginary plane of the roof but that stopped happening.

Is it to do with minor/major faces? What decides minor vs major? Can that staus be switched?

Encountering this in Shape but prob applies in Bricscad too?

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