Subsurf modifier seems to prefer a vertex for no reason I have what I thought to be a regular shape with quads only, but as you can see, there's a prominent "ridge" on one of the edges and I can't figure out why:


Here's the topology of it:


I thought maybe there was duplicate vertices so I did a "merge by distance", but nothing changed. Drawing a rectangle on top of one of the suspect vertices selects a single one as well, so I don't think that's the issue. The crease on all edges is also set to 0. Another peculiar thing is that selecting

Another peculiar thing is that loop selection doesn't go all the way around the object:


Here's the blendfile:
Going into edit mode, I see the shading is not correct.

To fix this, select the object, go into edit mode, select all vertices and then recalculate normals (shiftn).