I've had a Nokia 8 for about 1.5 years and since about half a year ago the Dual-Sight feature stopped working.
Mine works fine, but I did a factory reset when I upgraded to Pie. Your photos will all be saved to Google if you have set it up. All mine were available after the reset, although the images saved to Google are more heavily compressed. But if you want to keep your photos you can't possibly avoid backing up.I routinely manually back up my photos and videos to a harddisk via my computer, then check the Google backup is up to date, and only then, allow the photo app to delete photos and free up space.
Did you try reset the camera app settings - in the camera app 'hamburger' menu > Settings > reset to default?- Some combinations of resolution settings are known to cause issues in PiP mode, but dual-sight should work with default settings.
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