When dealing with iPhone files, it is important to properly color manage (assign and convert) them. Shared photos are converted to Apple Wide Color Sharing Profile, where a custom profile is generated so the data can be stored as sRGB but converted back to P3 for color managed devices. This is “optimized” for the Apple retina displays. The issue is this is proprietary to Apple. Apple considers forwarded, emailed or posted files to be “shared” basically whenever a file leaves the resident Apple “Photos” ecosystem.
So what is the solution? All Apple files ESPECIALLY SHARED ones should be assigned and converted to an sRGB profile such as Adobe 98 or standard sRGB (sRGB61966-2.1). Standard might look a bit flat because it is not wide gamut like Apple wide color sharing profile therefore Adobe 98 may yield a closer result to the Apple wide profile.
Be sure to convert and save Apple files prior to working on them. Some APP BASED PROGRAMS SUCH AS SNAPSEED, DO PROPERLY CONVERT THE FILES… SOME DO NOT.