Copied from a post by Francis Crossman, the product manager for Premiere Pro. Great advice, including how to switch off the function in your phone so you never have the issue until you are ready to embrace HDR.
Francis wrote: "One thing that is tripping up so many people here is that the iPhone is shooting in HDR by default, and it has an HDR screen, so videos look phenomenal on it. High Dynamic Range video contains more light and color level than Standard Dynamic Range video (Rec709). Unless you have an HDR display on your computer (and have everything is set up properly), you will never see it the same way as on the phone. The vast majority of people have an SDR display. When you send the video to your computer, QuickTime player will do tonemapping while sending it to your SDR display so it looks decent. Premiere Pro does not have this capability yet.
Here's what's happening. PPro reads the metadata in the file, sees that it's HDR (HLG to be specific) and treats it that way. If you create a sequence from the file the sequence will be set up as HLG automatically. But your monitor is physically not capable of displaying the light levels in the file so that's why things look blown out. If you look at the scopes, you will see that nothing is actually lost. You could use Lumetri to grade the file down to SDR levels.
Here are a few options that I recomend. Choose the one that works for you:
Don't capture in HDR on your iPhone.
On your phone- Go to Settings > Camera > Record Video > turn off HDR video
This will create Rec709 SDR clips
Edit in PPro like you are used to
OR . . .
Override the colorspace of the files to Rec709
Select all your clips in the project panel
Right-click > Modify > Interpret Footage > Color Management > Set the color space to Rec709
Now your clips will be treated as if they are Rec709 - this is not a conversion, it just tricks PPro into thinking it's Rec709
The clips will likeley look a little flat
Make sure your sequence is in Rec709 - Sequence settings > Working color space > Rec709
Color grade to taste with Lumetri. You will never get it to look as good as on your phone because your phone has an HDR screen and your computer (probably) does not.