Does Samsung change how brightness works in Android? I think since 7.x both Auto and fixed are adjustable. On my pixel, I have Auto brightness, but at the same time I can choose the baseline brightness level - it gets corrected from that point.
So the switch is really "fixed at point" / "floating around that point".
I just tried it on my GS7... sliding Auto Brightness from Off to On always bounces the setting bar down to baseline (about 40%) initially regardless of prior setting.
That's either old Android or Samsung breaks the behaviour then. You can have adjustable auto-brightness on stock Android. (Called adaptive brightness in settings)
So the switch is really "fixed at point" / "floating around that point".