Post processing + >100% Screen percentage results in color variation in part of image

As the title says – when I render out with a screen percentage higher than 100%, I can see what I can only assume are the chunks/buckets of the render


There’s a distinct line through the image where there’s a shift

I’ve tried turning off bloom, mega lights, etc

I found that the solution is just to completely disable all post processing. Which… is okayyyy because it’s for a film that’s being rendered and I can do the PP in literal post, but still wondering what else might be a solution

The render resolution is 4096x1716. The horizontal separation is exactly at 50% across (though it’s not pixel-crisp, there’s a couple-pixel overlap). The vertical separation isn’t at exactly 50% the image but my estimate is that it’s 1024 pixels down.

Any thoughts?