ThinInstances lighting behaviour seems off

[Evgeni_Popov]

I think this post should help you (but the entire thread is very informative):

Automatically Convert Instances to Thin Instances?

Here’s a working PG: The main problem was that the glTF loader sets the mesh.overrideMaterialSideOrientation property, which takes precedence over the material.sideOrientation property, meaning the new material side orientation was not taken into account.