I've noticed that the dimming at sunset sometimes becomes almost instant, rather than over a few minutes. It's annoying. The setting for transitions is still set to "Slow, Natural Timing". This rapid transition is even shown on the display, along with a weird rising pattern between sunset and night. It doesn't happen all the time, only occasionally.
I have a progressive delayed sleep cycle, so my sleep pattern changes constantly, and I spoof my location to get the timing I want from f.lux. Could this mismatch between "location" and the computer clock be causing this weird pattern? Ie, specific combinations of time-zone vs earliest-wake time cause the issue? If so, you might be able to repeat it.