It seems that the result is not correct when creating a Weld between a NonPlanar and a Planar face, which are not perpendicular. See https://cad.onshape.com/documents/7c7042b84e1ae5b391af796f/w/5ceb283e2e18597c2b7f644b/e/9e6a4df4db3a79d60e9a6d85 