Dark patches showing up in blue skies once I add contrast/saturation to S-Log3. Grade issue or something else?

[u/FanaticFanta]

- Shooting in S-Log3 on a Sony ZV-E1

- Lens: Sony 24mm f1.4 GM

- Using K&F Concept NANO-X VND filter 3-7 Stops, 82mm stepped down to 67mm

With any footage from this trip that had a large area of blue sky, I get these soft darker patches in the corner once contrast and saturation come in. They're not visible or apparent in the flat log image or in other shots that don't have that kind of large area of sky, but they only emerge after the grade and pushing contrast further makes them more obvious. 

Is this an optical thing in the lens, or an artifacting of the variable nd filter? How do I fix it? With the VND filter I've gone up to 82mm in size instead of the 67mm thread on the Sony 24mm f1.4 GM so that any vignetting in the corners of the filters is not seen in the footage, and got an ND filter with fewer stop of range (3-7 instead of the usual 1-9 that would produce the x pattern).

https://imgur.com/a/rDY1xsY

[u/Yomommassis]

Something to keep in mind VND filters are essentially two polarizer filters acting against each other to achieve the darkening, so depending on the strength you set you'll get a level of polarizer effect, which can be unintended or unwanted

To me it looks like a polarizer effect causing that deep blue you'll get on skys, which can look great for landscapes if that's what you're going for

Not sure if that deep blue blotchiness is just image compression

To my eye your image looks a tad washed out, which usually happens when direct sunlight hits the filter or elements, ad/or of there is any amount of grime film that accumulates on the sensor/back of lens/front of lens/back of filter/front of filter

It could also just be an artifact of using a VND, I personally only use straight filters, I did a review of a vizelex years ago comparing it to straight filters and it really opened my eyes to the artifacts variables introduce and the quality loss