Uneven screen brightness on 2012 Macbook Pro

Over the past year I've noticed that the lighting of my screen is uneven: there are two patches where the light is brighter. This is a lot more obvious with clearer, whiter backgrounds. Should be visible on the photo I've attached! The bright patches are roughly in line with F2 and F10 keys.

I have seen other posts describing uneven screen brightness, but the photos they posted didn't look quite like mine.

Any ideas? Late-2012 MBP with antiglare hi-res display. While it was under three-year warranty, I actually had the screen replaced twice before... first time because of a single green line which appeared intermittently, and second time just because the hinges were becoming wobbly. That most recent repair was 2016. Laptop is in excellent condition with absolutely no history of knocks or physical damage.