It is easily possible to show the Always On Display on Galaxy phones while charging through such option in Settings, but then you lose the ability to show it when tapping the display. Here is a quick tutorial on how to make it show up both ways.
So we have that somewhat useless Bixby button on Galaxy phones. Now I am not against Bixby, quite the opposite. But I prefer to summon her via voice and use the button for something I commonly need to happen fast - turn on the flashlight.