P2414H, dark left bottom corner

Hi. I'm a new to this forum, and please excuse me if I come off totally clueless. I have four identical out of warranty P2414H monitors. One of them has a dark shadow towards the left-bottom corner. I'm attaching pictures of the self-test pictures (gray, red, and green). I'm using multimedia preset, 75 brightness/75 contrast. I can still see what's on the darkened corner, but it's now quite annoying. I'm hoping to make use of these monitors (being used as two side-by-side pairs) as long as possible. I understand this is an old model manufactured in June 2014 (and I am not the original owner). Is there any solution other than replacing it (or all four of them)?

Thank you.