Rhino is not to blame for the way this is built, for sure. That would go to whoever at Chaos Group built this. But it’s definitely Rhino handling it in a weird way. It should either be invalid from the start or not at all, but moving it should have no influence on it.
What I find weird is that it only happens after a certain distance in Z. Could it be that the Z values of the Vertexes are like one significant digit apart and as soon as you move it above a certain value of Z, the accuracy basically makes it so that they then get rounded to the same value? Something like that?
Super weird. Can someone from McNeel take a look at this!?