A few of our users reported that images saved with our apps disappear from their library in Photos after a few seconds. All of them own a Mac with an old version of macOS, and all of them have iCloud syncing enabled for Photos.
Our apps use Core Image to process images. Core Image will transfer most of the input's metadata to the output. While we thought this was generally a good idea, this seems to be causing the issue:
The old version of Photos (or even iPhoto?) that is running on the Mac seems to think that the output image of our app is a duplicate of the original image that was loaded into our app. As soon as the iCloud sync happens, the Mac removes the image from the library, even when it's in sleep mode. When the Mac is turned off or disconnected from the internet, the images stay in the library—until the Mac comes back online.
This seems to be caused by the output's metadata, but we couldn't figure out what fields are causing the old Photos to detect the new image as duplicate. It's also very hard to reproduce without installing an old macOS on some machine.
Does anyone know what metadata field we need to change to not be considered a duplicate?