Avoid polarizers on wide-angle lenses
You'll get weird effects with a polarizer on any ultra-wide lens. Linear and circular polarizers give the same weird results on skies.
The weird results on ultra-wide lenses are because the polarization of the sky varies from one end of the sky to the other. Polarization of the sky is maximum at 90 degrees from the sun, and none at the sun and 180 degrees from it. This is no issue with a tele lens, however with a super wide you are sweeping such a broad expanse that the polarizing effect will vary greatly across the sky. You often get a dark area in the sky, which usually looks nasty. I rarely use polarizers with ultra wide lenses.
If your polarizer is thick (as many are due to the rotating mount) you may be getting mechanical vignetting in the corners, too.