Summary

It seems to happen only on a specific geometry class (a geometry plugin that I develop). The only thing I can think of that might be different to regular objects is that it is a scripted geometry class with explicit normals. I tested converting to editable poly and the error goes away.

Yes, I can do that. I'll get you the needed files this week. It will require you to have a machine with at least one Source Engine game installed (such as Half-Life 2, Black Mesa, Counter-Strike: Global Offensive, etc). The plugin reads the game engine's native model format to load directly into Max--and it is this plugin that I discovered the issue. Note I'd not likely use these geometry types in a real life situations--I just happened to test OX6 in my default scene which has one of these nodes by default. But I think it would be good to know how/why this is happening.