Postprocess after Mesh.TopologyEdges.CollapseEdge()

Hi, I tried cleaning up a bad mesh which contain extremely short edges.

I tried calling Mesh.TopologyEdges.CollapseEdge() for all extremely short edges.

This resulted in a "Invalid Mesh".

I created a new empty mesh and copied all faces and vertices. I got a good mesh.

I wonder what's the difference of the mesh after calling CollapseEdge() and the one built from a scratch? What sort of post-process I need to do after calling Mesh.TopologyEdges.CollapaseEdge()?

Thanks you.