It changes but it does it slowly, I thought mine was not working too the first couple day and was ready to exchange at the end of the month.
Besides I think auto brightness uses PWM which causes flickers at a very high refresh rate not viable to human eye but causes fatigue to the eye over extended use and the gamma changes too much.
I now just turn it off and set the brightness to my liking depending on the lighting conditions, it's easy enough with the touchscreen and fast menus.