The colors show up in a shaded view, but not in a rendered view. STEP import assigns object colors to the surfaces, but not materials. Maybe @pascal has a way to transfer the colors to materials. I don’t know anything about rendering.
To fix the geometry problem, isolate the pair of surfaces that use the ugly edge. Call UntrimAll, keeping the trimming curves. Repair the curves by exploding, splitting off the big loop. adjusting what is left to join up, and then re-trimming the surfaces.
My V6 code to automatically fix this problem should have worked, but some low level intersection code was not behaving as expected. When that gets fixed the import should just work for you. Thanks for the report.