Hi,
I know this is an old post so hopefully you figured out a solve for this issue but I have recently been experiencing a similar problem and wanted to post my discovery to potentially help anyone in the future.
When I would open a photo file be it a jpg, png, cr2, etc. it didn't seem to matter, I would get this automatic refresh of the image within a second or two and it would darken the whole image and almost seem to reduce the overall quality of the shadows. For a brief second I would see the image how it was supposed to look but then every time it would do this darkening process. I could open it in photoshop and this effect wouldn't happen so I knew it was something to do with preview or the 'quick look' feature on Mac.
I started messing with the settings in preview and found a feature in the dropdown menu bar:
Tools > Assign Profile
When you click this you can change the ColorSync profile on the image. So I would select to change it from "ACES CG Linear" to "Existing Profile in Image". This would revert the image back to the original look but then it would change back again after a few seconds and also change back in to "ACES CG Linear" in the Assign Profile dropdown.
I think I have found a fix for all this but will update if it returns. The solve I found was to change the "Display Profile" in
System Preferences > Displays > Color
by clicking the "Calibrate" button and making my own display profile has removed this darkening effect to all my photos in Preview. The ColorSync/Assign Profile automatic reversion still occurs but doesn't doesn't seem to have any affect on the image.
I still don't fully understand why any of this is happening but maybe this is the fix... I don't know.
Hopefully this helps someone!
- George