I usually select the vertex, grow selection, then use Detach component in vertex mode to split all connections. Then select them again and use weld vertices with a low threshold. It should work, but then you have to redo your soft/hard edges and sew UV's.
It could be similar to this recent thread. It could also be a set of faces have backward normals too, Mesh Display>Conform Normals. Box select that vertex and maybe try its neighbors too, check the HUD to see if there are multiple verts, edges or faces there. Or check in smooth mesh preview to get a quick look at how…