9 pages of this stuff, wow.
This is normal behavior for an OLED display. It differs on each phone depending on how it is used prior to the gray being displayed at very low brightness. It's basically image retention on a very minute scale that's imperceptible except on that one shade of gray at a certain brightness under 15%. When the sub pixels are tasked with doing gradations at rock-bottom brightness, you're asking for accuracy when they're barely being fed enough current to stay powered on and depending on what those sub pixels have been displaying prior, that same current will flow differently through them. This is well beyond any normal usage scenario.
This does not affect day to day use or overall quality and if you're really bothered by seeing it for five seconds in the clock app at 1am, you should re-evaluate your concerns because this is totally normal for this display tech and professionals have been using multi-thousand-dollar OLED reference monitors on film, TV, and commercial sets for years that would exhibit this issue if tested specifically for it. It doesn't affect the usefulness, accuracy, colors, image quality, or benefits over LCD at all as much as you think it may and clients often sign off on shots for $500,000+ budget productions using these monitors as a reference.
If you think there's a problem, go back to LCD and enjoy backlight bleed and glowy blacks. Otherwise, enjoy the phone. It's one of the best displays on the market.