iPhone Auto-Locks while charging; it didn't do this previously

You may be right. But I've had this iPhone 6 for over 2 years and it's never Auto-Locked while charging through all the previous updates during that time. In fact, I'm on my second iPhone 6 (my first one was stolen a few months after I got it), and the first one never Auto-Locked while charging either. Also my previous iPhone 3Gs, which I owned for 6 years, never Auto-Locked while charging.

I've always listened to music at my desk as I described above, and I've never noticed any special setting to differentiate between charging and not-charging, but I've always consciously thought how much I liked that it doesn't Auto-Lock while charging.

Weird that we've had such different experiences. The only time my iPhones have Auto-Locked while charging is if I went to the home screen. But if I had any app, like the music app, on screen, it would stay awake indefinitely while charging.

By the way, I've checked to see if there's any setting in the music app itself that would keep the phone awake while the app is active (like it does with Waze), but there doesn't seem to be anything like that.
I really only want it to stay awake while the music app is on screen. That seems like a fairly obvious app for that to be an option. But maybe I'm biased.

Is there any chance that the Music app used to have this feature, and no longer does?

In any case, there has to be some explanation of why my phone behaved differently, even if I can no longer get it to behave that way. And, as I said, I've seen a few others post questions about this very same issue, so I know I'm not the only one whose phone did this (although perhaps it was rare).

Nope, no new email accounts. The only change I made was installing the 11.2.2 update.