Well, I had never seen that setting before, and that's a very interesting one to know about. But do this for me (and by the way, thank you for helping me with this!!!)....
Take any of your photos and open it on your iPhone. Click edit. Click the Auto button and let your phone auto-fix the photo. Now, take notice of the 3rd icon over for Brilliance. Maybe click on it and slightly adjust it just just a tad, enough to see the actual number setting it assigned your photo. At this point, you know your iPhone adjusted the brilliance setting (I like to adjust it even more most times). Anyway, don't make any more changes, don't crop, and don't fix the photo any further. You can scroll through the various other settings and notice that your phone did make other settings changes, probably to things like Brightness, Contrast, and Saturation.
Now, use AirDrop and send it to your Mac, and use that setting you told me about. Once the photo is on your Mac (my AirDrop sends the photo to the Downloads folder), drag it in to your Photos app. From the photos app, open the photo, click on Edit in the upper-right. Now take notice if the Brilliance setting copied over. On my Mac, it does not. In fact, the photo appears to have lost ALL the settings that my iPhone's Auto fix feature seemed to apply to the photo. Settings like Brightness, Contrast, and Saturation have all seemed to reset back to the original setting for the photo.
Thanks again!
Jan