Clouds lighting often doesn't get updated properly, as shown here when switching the time from midnight to day, and then switching it to night:

Note that the same issue can occur during normal gameplay, and the /time command isn't required to reproduce this issue.

Screenshots were taken in an instance with forge 43.3.5 and BadOptimizations 1.6.3, and I had no other mods installed.

Turns out this applies to all forge versions but still doesn't appear to be an issue on fabric.

Odd, do you have any other mods running?

Nope, just BadOptimizations.