One bottle oxidized, the next one fine. Why?

I've been struggling with what I believe is an oxidation problem with my IPAs off an on. The first 1/4 - 1/2 the batch that I drink in maybe the first 3-4 weeks is great, and the rest seems sweeter, "maltier" sort of, and the color has darkened. But for the first time I've seen the change occur from one bottle to the next.

The bottle change was after the switching from the end of the 2nd of 4 12 packs to the beginning of the 3rd. The last bottle seemed oxidized, but the next one is fine. I don't know the order of bottling so it's possible I could've been switching from the first to the last bottled, or vice versa. But I'm pretty sure I did adequately mixed the priming sugar. They were stored together, the 1st and 2nd 12 packs on top, too close for a temperature difference but the top 2 may have had a little more sunlight on them.

Note the difference in color in the pictures. The taste difference isn't quite as drastic as the color difference at this point.

This was bottled on 2/21, I just took these pictures today, 5 weeks later. Should oxidation really happen that quickly? Any explanation for why it could affect some of the bottles more than others? Is it something else?

 

Those are two glasses from two bottles. Didn't have a third hand to hold a camera and two flashlights so I took two pictures. Without the flashlight the color difference isn't as obvious on camera.

Here they are side by side:

 

No, can't say I ever thought to describe it as vinegary. And I wasn't actually noticing a huge difference in taste between these two bottles, but in the past there seemed to have been a correlation between the darkened color and the sweet/"malty" taste I was experiencing.

I believe the caps were on tight, the carbonation level was about the same so I don't believe any gas escaped. And I don't see any cracks in the bottles.

I soaked in oxiclean but I doubt there was any residue left in, I gave them a good rinsing after. Then I soaked in StarSan, it's possible/likely there was some sanitizer residue left over.

Could bottle cleanliness be a factor? Maybe the oxiclean soak in this batch wasn't long or hot enough, or maybe it was too diluted, and some bottles were left cleaner than others?

 

I've begun a test for this. I've chosen 2 bottles that look identical at the moment (the two sealed bottles involved in the above pictures were clearly a different color, which is why I opened both at the same time). I'm leaving one exposed to diffused sunlight in a well-lit room, where I keep most of my homebrew. The next is sitting right next to it but covered, in total darkness. I'll leave them for a few weeks and report the results.

In the meantime I've thrown a dark blanket over the rest of my beer in case this is in fact the culprit.