I finally got this to work, but there is definitely some inconsistency with the USB-C port of iPhone 15 and the Apple Adapters.
First, I didn’t actually do anything other than to unplug and replug the adapter a few times for DAC to be recognized. This rears its head when I stop the music for more than a minute or so and then try to resume play. I’d have to start unplugging and repluggjng the adapter before the DAC is recognized.
Second, because I had turned on the “Automatic Dolby Atmos” setting, the phone was always outputting in 44k, so I mistakenly thought that hi-res wasn’t working. Turning Atmos off subsequently output hi-res stream.
Third, in iOS 17, unlike iOS 16, the volume rocker component lets you move the volume setting, but it doesn’t actually affect the output volume. This was throwing me off too, because I was expecting iOS 16 behavior where volume is set to max and you couldn’t change it.
Fourth, I have one DAC that has a dedicated iOS setting and didn’t require the Apple Camera Adapter. That used to work and made the phone go into that fixed volume mode in iOS, but now it doesn’t happen, and both the iPhone and DAC/Amp volumes are functional. I think this means the output is not hi-res digital. I’m working with the manufacturer on that one.