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You can choose a subset of photos, with an option to select more photos whenever the app tries to access your photos, or you can grant the app full access to your photos.
For example, when using the "Selected photos" option with Instagram, swiping left to post a story would present system popup asking if you wanted to add more photos to your preselected set, or keep the ones you've already selected in the past.
As of Instagram's update 2 days ago, when you swipe left to post a story, or when you go post a regular photo, you no longer get prompted to select more photos or keep the current selection. This still works with other apps. This makes me believe that Instagram is somehow not trying to access the photos library when the user has chosen the "Selected Photos" option. I don't know how they are detecting this (Maybe using an Apple internal API they've discovered?)
This makes this privacy feature unusable for end users. With Instagram's hack, in order to give the app access to more photos (Say if you just took a new photo), you have to go into Settings -> Privacy -> Photos -> Instagram -> Edit Selected Photos every time you want to add more photos.
I'm sure users will opt for just granting access to all photos instead of having to do this.