I'm an insider running the 18836 version.
Using f.lux 4.96, the colors are changed according to the time as before the 19H1 but the brightness control still does not work.
I see the "Dimming until sunrise" popup with a circadian response percentage but it does not change anything. Is it planned to be fixed ?
NVIDIA 188.8.131.525: NVIDIA GeForce GTX 1080 (20181211)
Generic PnP Monitor, 621 x 341mm, built 2017
sRGB gamut: 91%, AdobeRGB gamut: 64%
Windows Build: 18836