This appears to be a deliberate and inexplicable change made by someone at Apple. Portions of the left and right edges of all artwork are being shaved off, leaving a rectangle that's taller than it is wide.
In the attached example, there should be a white border around the entire image. But this change removes part of the left and right sides.
I was a part of the beta-testing process from day 1, earlier this year, and the moment I saw this, I reported it as a bug and provided Apple with screenshots. I subsequently re-reported it a few times and pleaded with Apple to reverse this change, but it was never fixed. (To provide context, dozens and dozens of other bugs I reported were fixed over the course of the 3-month testing period. But this artwork issue was never addressed.)
With Apple's decades-long reputation for attention to detail and reverence for art in all its forms, I'm forced to believe that someone at Apple did this intentionally—and it was ostensibly approved. But it makes no sense. Album covers have been square ever since they were invented, which was what? 60+ years ago? And I've curated my album covers in Apple Music (formerly iTunes) for more than 20 years. But now, every album cover is rectangular.
It's utterly, utterly heartbreaking.