On recent iPhones the always-on mode does not switch the wallpaper off; it dims it heavily and drops the refresh rate to one frame per second. Many Android phones take the other route and show a clock on pure black, with perhaps one pixel in twenty lit. Either way, the lock screen you chose in daylight spends most of its hours in a much darker version of itself.
Dimming is not even. The bright parts of a picture keep their shape, while the lower tones sink together into one flat dark, so a dusk sky that looked rich at full brightness can turn into a single muddy band. Images with one clear subject in the middle tones, a pale flower or a lit window against a deep background, hold up better than scenes built on subtle shadow detail. The clock also sits on top for every one of those hours, so a calm upper third matters more here than on any other screen.
If a favourite photo falls apart in the dim state, iOS lets you keep the always-on clock and turn the wallpaper itself off in the display settings, and many Android phones let you pick a separate image for that mode. It is worth checking the phone face-down on a desk for a minute. The version you see from the corner of your eye, at one frame a second, is the one you will look at most.
