when dealing with normals
- we know we typically have issues for models having a "mirror modifier" where typically the result ends up having inverted normals
- for all geometries that have a single null normal, we will end up recomputing all normals (for the corresponding geometries) because we don't know if this is intended or a "bug"; if you want an unlit part, you need to put small but non null normals. This is constraining but providing a nice UX to control normals (locally) is difficult so this might not be implemented in the near future (if at all on sketchfab).
Does this apply to your model?
Also, I'm not sure why you'd need to "relight" your model after a reupload? The reupload feature is supposed to re-apply your settings. If it doesn't, it could be a bug. Could you provide a bit more details about your workflow here?