I don't know if it's part of Marshmallow or TouchWiz but now the brightness slider controls the automatic-ness of the brightness. So its more like a auto "range selector" or "sensitivity selector" it seems. Its more of a semi-automatic mode.
If you set it to ~50% the brightness will adjust +/- 10% around that setting. But it doesn't seem to just automatically do 0-100% on its own anymore.
Set it near 100% and it'll practically refuse to dim in low indoor lighting.
It also seems to respond to whether or not you're holding the phone. It'll get brighter no matter what (thankfully), but it will not lower the screen brightness until you put it down on a horizontal surface... Kind of annoying if you're reading and happen to walk in to a dim room and you're being blinded by your phone until you put it down and then pick it back up. I have Smart Stay turned off so that's not it.
I definitely prefer the plain old, dumb, just works, auto-brightness that I could snap on and off if I needed. Now its...complicated. Why "fix" what isn't broken. Was it confusing some people?
I tried Lux Brightness control app but I'm not sure it's really what I'm looking for. It's too much. I just want "set it and forget it" Automatic (0-100%) brightness like it used to be.