When "View Full HDR" is turned on in the settings, it simply means that the out-of-camera photos in your Photos app will look different in the Photos app than they will anywhere else that you view them. It's altering the way they look in the Photos app only. Turning off "View Full HDR" will allow you to see photos in the Photos app as they would look in other apps, such as in Lightroom Mobile or even in Instagram. If the photos that you take on your iPhone are going to be viewed anywhere other than YOUR iPhone -- like in Instagram, or texted to friends, or sent to your MacBook, or printed, than you'd be wise to turn off "View Full HDR."
Thanks for the info.
I've just upgraded from an iphone 6S to 15 pro max and could see something funny was going on; the 'iphone' photos look amazing - until you import them into LR mobile.
Also, I had turned off video HDR (which is under camera settings), and you could see the difference between the photos and videos in the iphone photo library. Now I have turned off HDR in photo settings, and things are much more consistent [especially when I import to my desktop!]
So, some new info: In LR mobile under app settings > Import, there is an 'HDR edit mode for new photos'. This processes HEIF/HEIC and JPEG images in real time as you open them - with an RGB histogram showing this happening. Doesn't work for DNG's.
For the time being, I'm turning it all off until I have read up on it all.
[Another thing: Make sure you have your colour space warnings turned on so you can keep track of things when using Bridge / ACR / LR / PS etc. iphone jpegs and HEIC/F's seem to come with the P3 space].