Sometimes parts of 1st-party apps will get weird vertical banding across parts of their interface. I feel like I see it most often with Music, but I see it regularly on the text input panel in the Messages app. It goes away once you give the app focus and apparently doesn't show up in screenshots.
It first started showing up when they added that thing where the side panels on the first-party apps that don't have transparency but do get slightly tinted by whatever the dominant colour behind the section is so I assume it's related. It's fun that it happens to components that do not actually use that feature