Cannot clear seam from individual edges

Question

After modifying a mesh with an existing UV unwrapping, I am unable to clear seam from some individual edges. In the screenshot below, I created the leftward section of the mesh after the initial UV unwrapping by subdividing a face manually with the knife tool and then extruding it. After this, I wanted to clear seam from the highlighted edge (white) in the screenshot so that re-unwrapping the mesh would produce a result continuous with the pinned UVs on the face to the right of the highlighted edge, but clicking "clear seam" with the edge selected does nothing. Most other seams on the mesh can be cleared individually. If I select the entire mesh and click "clear seam," all seams are removed.

Is this inability to remove seams intended behavior, and if so, what rule justifies being unable to remove individual seams like this one? In this question, the inability to clear seam was due to non-manifold geometry, but this mesh should be a manifold. The Blender docs make no mention of clear seam being an operation that can fail, and no error is displayed in the interface when trying to clear the seam.

Answer 1

While writing this up, I thought to check for any degenerate geometry that may have been produced while extruding the face. Sure enough, after applying Mesh > Clean Up > Degenerate Dissolve, I was able to clear seam on this edge. There was a degenerate face between its two endpoints.

It would still be nice to know what rules affect when seams can be cleared. I find it surprising that this isn't mentioned in the docs.