GH error when Join

There is a problem here, I have 2 surfaces, over which I transform (flow) some surfaces, and as a result we have a group of surfaces (2 pieces) that I connect. As a result, I get a polysurface (solid, 0 open edges) with irregular normals. But not always! Changing the tolerance does not help. If Explode, and then Join, then everything is restored. :slightly_frowning_face:
P.S. I can not attach the GH file, and it is unlikely to help here.
1.3dm (758.0 KB)