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.