On both Samsung phones and tablets, I've found Auto Brightness to be unreadably low at first and had to shut it off. (With Auto off, you can set whatever fixed brightness you want, it's not a binary Auto vs. Max choice).
I have the GS3, GS5, GS6, GS6 Edge, GS7, GS7 Edge, and a Tab A.
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)
I have the GS3, GS5, GS6, GS6 Edge, GS7, GS7 Edge, and a Tab A.