Draw mode "Solid (colored per vertex, shaded)" actually shows colors per face

After assigning a color to each vertex and switching to the draw mode "Solid (colored per vertex, shaded)", I see the following:

Instead of linearly interpolating the vertex colors, constant colors are shown per face.

The non-shaded draw mode "Solid (colored per vertex)" works just fine:

(Exact same colors are used)

In both cases, the shader pipeline renderer was used.