Thank you for creating a bug report and, in turn remaining patient with me. 
Just to clarify, the original seams were also a result of the incorrect normals occurring due to non-uniform scaling. It’s just that when I posted the question, I hadn’t come to the conclusion that this was the problem.
Basically, this entire thread is just different manifestations of the same underlying problem.
I will avoid non-uniformly scaling instanced meshes for now, but if implemented, this will be a really powerful feature, allowing batched rendering of architecture (walls, panels, etc.) instead of just using it on foliage/props.