I am having the same problem. I used 3 cameras to create a "Ski Training" movie last march. Now as I finally find time to edit, the iPhoen 12 Pro clips are "blown out". I am now aware it is becasue of 10 Bit Dolby Vision HRD and that it nees to be downgraded to work with my older 4K cameras.... HELP????
The way I finally found to deal with this (thanks to this video: https://www.youtube.com/watch?v=WKAEtl0E4T8) is to move the HDR video from the iPhone to the Mac. In the Finder, control-click on the filename and select "Encode Selected Video Files". From there, choose the format and resolution. It will process the video and create a new SDR version, which can be imported into Premiere.