By blank union I am referring to an empty union created by Instance.new("UnionOperation").
What happens appears to be that the scale of the FileMesh is ignored and the mesh renders to the size of the union. Is this volatile behavior that is prone to change, or can I rely on the behavior of meshes parented to unions?
Why use a Union instead of a MeshPart?
And the other question is why load a blank Union or MeshPart to then put a File in it?
Loading the asset dynamically is not possible with MeshPart
You should explain why you want to do it this way.
Most people would have the complete MeshPart in ReplicatedStorage and just clone it from there.