got the same problem, worked once and now it's greyed out, also on a fresh install with usual changes in the settings.
it worked but I did it with little different. first got a backup of both below two reg keys, than I deleted both keys, restared and it worked. than checked again to make sure those two reg keys reseted, seems like those keys automaticaly created again but nothing to worry night light working as it should. Spoiler: Night light Reg HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\CloudStore\Store\Cache\DefaultAccount\$$ windows.data.bluelightreduction.bluelightreductionstate HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\CloudStore\Store\Cache\DefaultAccount\$$ windows.data.bluelightreduction.settings