I think I figured this out.
I had the screen dim a couple of months ago while I was watching something on Apple+ in my home. The phone has a case on it and felt warm. I took the case off and the phone itself was hot. about 5 - 10 minutes later, the phone felt cooler and the screen brightened back up.
I've been watching for the screen dimming on its own and the phone was fairly hot each time.
The phone's heat dissipation properties are probably not engineered to consider the various types of plastic or silicone cases we're using. They are essentially insulation that impairs the phone's ability to stay cool.
The solution here may very well be to remove the case when you need brightness all the way up and the screen has dimmed on its own for no apparent reason.
Hey Apple! Maybe you could let us know if this is the case and, if so, make this feature more public. Like: "The phone screen will dim if the phone overheats. Phone cases can impede the phones ability to stay cool".
Love to know what everyone else has discovered. When this happens, take it out of the case. Is the case hot? Does the screen brighten back up after the phone cools off?