This post from Android Police explains how the adaptive brightness works:
In contrast to the auto brightness in older Android versions, the display does not change its brightness in the full spectrum (from 0 - 100 %) anymore, but only in a range around the selected brightness value. This should allow the user to set a base value, which yields the most comfortable display brightness in different situations. Unfortunately it seems that the developers forgot that users might not only want to set a bias, but also the range for the brightness adaption, because they fixed the brightness variation range to a fixed value.
How this works can be observed when the device is in a dark surrounding. If the brightness thumb now gets dragged from low to high, there is a rather long interval, where nothing happens. This is because the algorithm, which sets the display brightness, wants to set it to the minimum value. As long as the slider thumb is no more than the fixed brightness variation range away from the 0% mark, this means that the minimum value is 0%.
As soon as the slider thumb is further than the fixed brightness variation range away from the 0% mark, the display brightness increases, because the lowest allowed value for the algorithm is now at the lower border of the brightness variation range. Because this border is moving together with the slider thumb (unfortunately not visibly), the brightness increases.
This means that setting the slider thumb to either the minimum or maximum value yields a loss of brightness adaption of 50%. To have the maximum adaptation, make sure that the slider further than the adaption range away from 0% or 100% brightness.
In a quick test, I observed that the brightness begins to increase at a value of about 50% in a dark surrounding. This would indicate that the brightness variation range is about 50% around the selected slider value. Thus the middle position of the slider should yield the best adaption range. If you want to get rid of the display flickering, set the slider a bit above 50%. Then the brightness never gets into the critical area.