Equirectangular environment texture: unexpected interpolation on pole (cycles only)

Interpolation applied by an environment texture seems to mix north-pole pixels and south-pole pixels (see attached image). Eevee doesn’t have this behavior, so I want to know if this is something expected or known limitation of the cycles (or a bug that I should report).

Note: The scene is set up to exaggerate this behavior, and I think it is not a big issue in practice (because usually we use a large equirectangular image instead of 6x3 here).

Thank you very much.

blender version is 3.3.0 on Mac OSX