But shouldn't the app be programmed to maximize the chances that it'll work on the first try? It seems like a reasonable compromise to me. Annoyingly bright for a few seconds so I don't have to futz with my brightness.

No, you cannot manually turn on HDR on iPhone.

I can turn the brightness up myself if the QR code doesn't scan.

Well, you're one person. How about the other eight billion people on the planet?

Everyone is not you.