Usually this is due to video drivers - try updating drivers, or post f.lux options > driver info here.

Please turn off Windows Night Light.

now when I look at it I have this feeling that lowering gamma to minimum with QuickGamma makes colors washed out, but dark, but when I enable f.lux color are washed out but brightness goes way up

Getting the same issue with Dota Underlords.

"there is a feature that disables f.lux for fullscreen apps, so check if you've set it in f.lux's disable menu."

This was it, thank you so much!

I figured out the issue. Windows 10 has a new "feature" called "Fullscreen Optimizations", which forces every app into borderless windowed mode (!?!). This also explains why all of my games have been getting significantly worse performance since the update.

I haven't figured out a way to disable it globally (none of the suggestions online work), but you can disable it on a per-app basis by finding the executable, right-clicking --> properties --> compatibility --> "Disable Fullscreen Optimizations". After doing this, flux will disable correctly for that application.

Turn off the setting called "Software mouse cursor when needed".

I've got the exact same problem, can't use the dimming feature as well. It's because of the latest 1903 windows update, nothing else. Hope this gets resolved soon!

@herf Ah, just that. Done. Sent. Let me know if there's anything else that'll help you narrow it down.

@nikku I got the latest version as of June 06th 2019 v4.104 and it seems to do the same thing since the May 2019 update for Windows 10 :/

yeah after windows may update this problem happen for everyone
look here
https://forum.justgetflux.com/topic/7021/windows-10-may-update-1903/19

still no fix :(