Thanks, it took a while but I am now looking through the Blender exporter stuff I put on ice. I did almost the same thing with a vertex.co.copy(). Positions & normals are stored as objects in the first pass. After all the meshes are run through, then in another pass, they are written out.
These meshes are actually temp meshes, where the modifiers are applied. An Armature is a modifier, that is the tie in, I guess. When there is no modifiers, it must just use the permanent mesh data. I now have to actively “clear” the temporary mesh in 2.80, which I do. That probably should have been a clue that I cannot rely on the objects still being there later.
This minor change will be on the next file.