Reading up on your previous thread, Menno seems to suggest that there is a difference between the render mesh and the viewport shading mesh.
So if by default the render meshes are not generated for viewport shading, they will only be generated once you start to render. Then they can be cached and reused for the next render. If you save a large file, they are stored in the file. If you save small, they are thrown away. That’s basically, what you have seen.