Fig. 1 max, Fig. 2 min of screen brightness where you see no difference
Bug hypothesis in CM13 discussed with the authors here: no manual adjustment of brightness when the system auto-brightness control is on, because its what VAB does when you enable VAB's function.
Thread in CM13 forum here for evaluation.
Phone: Oneplus 2
OS: CyanogenMod 13
Su: no
I can't comment on the ROM behaviour on your device, but here is an alternative that is found to fix the problem - see OP's comment
Velis Auto Brightness, app description of relevance says:
Velis auto brightness aims to provide the best possible brightness experience by using your devices sensors to determine the environment you're in. You have complete control over how much brightness will be applied for any given light condition, from selecting the sensors used to fully customizable brightness graph. This is a replacement for system provided auto-brightness functionality usually found in Settings / Display / Brightness
After installing app, OP is able to manually adjust the slider for required brightness
Q & A