Screen is darker on top right section

So this is a new issue for me. I have replaced several iPhone screens including the iPhone 6 Plus, but this is the first time I have seen this:

Phone was water damaged and stuck in an Apple boot loop. First thing's first, with a water damaged phone, I removed all components, including EMI shields and gave it a good cleaning in an ultrasonic cleaner. Checked the phone for any blown components, shorts, etc. Everything checks out to the naked eye and a magnifying glass.

So when I re-assemble everything, phone is still stuck in boot loop, but that was resolved when I restored the device through iTunes.

Now the problem I am experiencing is that there seems to be some inconsistency with the backlight. The upper right corner of the screen is darker than the rest of the phone. The issue that appears is similar to this image:

http://www.techwarn.com/wp-content/uploa...

I thought that this might have been from the water damage, so I tried a working screen from another iPhone. Same result. And after attempting a third screen (this time a brand new screen), and obtaining the same results, this leads me to believe something is wrong with the motherboard.

Has anybody had this issue before or does anybody have any advice on where to begin with this problem? Any help would be appreciated. Thank you!