A straight forward enough task one would think 
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This problem hasn't been specific to a single model of iphone, its been consistent across 11, 12 and now a 15 model. I change from the lock screen, or from settings > wallpaper but no matter what, it always defaults back to an old photo after an (unknown) set amount of time. I don't want to delete the photo as its an important one to me but I just cannot get the setting to save.
.This problem hasn't been specific to a single model of iphone, its been consistent across 11, 12 and now a 15 model. I change from the lock screen, or from settings > wallpaper but no matter what, it always defaults back to an old photo after an (unknown) set amount of time. I don't want to delete the photo as its an important one to me but I just cannot get the setting to save.