Trim surface with triangles

Hello!
Can someone explain to me why the region difference does not run as it should?
Trim_surface_triangles.3dm (83.7 KB)
Trim_surface_triangles.gh (12.2 KB)

I cant explain why it wasn’t working, but i could get it to work.
Trim_surface_triangles_WCPM.gh (15.8 KB)

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Trim_surface_triangles_re.gh (12.7 KB)

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Another time, thank you so much!