3DS Max Exporter - Mirror Geometry Normals Inside Out when Exported[Bug]

When you mirror instanced / referenced geometry in 3DS Max then export it using the latest 3DS Max exporter to gLTF, the mirrored geometry in Babylon appears with backwards facing normals while the source geometry has visually correct normals.

It sort of makes sense in that when you use Max’s mirror feature it basically just applies a -1 scale, which sort of would turn the geometry inside out. The fix may be tough as this may be as much of a modeling technique / 3Dsmax issue as an exporter issue, but I figured I would report it anyway.

I am using Max 2019 with latest Exporter.

Ping @Drigax and @PatrickRyan

Hmm, I’m not able to reproduce this bug on my own:

I have a simple scene here:
Simple Shape with Mirror Modifier.zip (61.1 KB)

that has a single cone split in half with a mirror modifier applied to recreate the other side. On the left is the original mesh and its mirror, while on the right is an instance of that mesh

Loaded into the sandbox, its normals look…normal.

Can you help reproduce the issue?