Models uploaded to Sketchfab without valid vertex normals will undergo Sketchfab’s normals recomputation where edges will crease when the adjacent faces form angles greater than or equal to 45°. We will check into this problem further and provide a fix in the near future. For now you work around this issue by:

1) Export the model in FBX
2) Importing it back into Character Creator 2
3) Upload to Sketchfab

For more relevant information: https://help.sketchfab.com/hc/en-us/articles/209143406-Vertex-Normals