Irregular polarization with EF 35mm f/1.4L II + Drop-In Filter

[ironlawn]

I recently bought Canon's EF-RF mount adapter with the drop-in CPL. As expected, it provides the same effect as a front-mounted CPL with my:

- Canon 16-35mm f/2.8L III

-Canon 11-24mm f/4L

-Canon 300mm f/4L IS

But when I use the filter with my Canon 35mm f/1.4L II there is very irregular polarization of blue sky. The filter cuts reflections off of surfaces like normal, but instead of the usual smooth gradient in the sky produced by a CPL I see an odd X-shaped darkened area in the sky at max effect that becomes more defined upon stopping down. Wide open it's hard to tell anything is wrong but at f/2.8 it's somewhat noticeable and by f/4 it's very obvious. Photos with significant sky are totally ruined. Rotating the filter lessens the effect but there are still irregular and disjointed areas of polarized sky that look terrible. If I remove the filter from the slot there is no issue, and if I used the lens with a standard EF-RF adapter without the drop-in filter slot it works nominally. I tried the combo on an R5 & R6, with and without the lens hood, absolutely no other filters attached to the front of the lens, no lens corrections turned on, and nothing seems to solve the issue. Has anyone else used their 35mm f/1.4L II with the drop-in CPL? I'd like to confirm it's not some sort of compatibility issue before considering service or returning the adapter. I hesitate to send either the adapter or the lens in to Canon since both seem to work flawlessly when not used together. 

[rs-eos]

I had thought the X pattern only showed up in variable ND filters.  Though I think such filters are created with two sandwiched polarizing filters.  I haven't seen this issue with CP filters.

Having said that, are you saying this issue only occurs with the 35mm lens? Assuming you took photos of the sky at the same time and same angle in relation to the sun with your other lenses?

[ironlawn]

Yes, this only occurs with the 35mm lens. I have indeed tried shooting the same photo from the same location at the same angle with my 16-35mm f/2.8L III at 35mm and the filter works fine with that lens.  

From what I've seen variable ND filters cause a similar pattern, but it affects the entire frame whereas this only affects the sky.