On my Galaxy S5 Active, why does my custom lock screen background disappear occasionally?

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AC Question

Galaxy s5 Active: custom lock screen background disappears occasionally

I have a photo set as my custom lock screen background, but sometimes when I turn on the screen, the photo has disappeared and it's just a black background. Lock screen and phone still function normally. Sometimes this issue fixes itself, but it's been a few hours how and I miss my custom background! I've tried restarting the phone, and also resetting the background using both the "wallpaper" option in settings, as well as "Set as..." the shortcut in "Photos." Still black.