Currently vanilla game sky colors impact deferred rendering shadows, which makes it almost impossible to get good looking shadows without making Biome sky colors black or significantly darker. 

In screenshot "(example pack) normal sky.png & normal vanilla sky color.png" is with default vanilla sky color, second screenshot "(example pack) black sky & sky color black.png" is with biome sky color set to black which makes shadows look a lot better. 

please watch the unlisted YT videos it shows it a lot more visually
Demonstration with included example pack
Demonstration with custom pack - YouTube

observed: vanilla sky colors cause deferred rendering shadows to be slightly over saturated and brighter than they should, in order to get shadows to look correct you need to make Biome sky colors darker in your resource pack.

Expected: the shadows should stay like the second screenshot unless edited by color grading file.

reproduce: import the included pack and find the edge of a normal Birch Biome. You will see that the shadows are affected by the Biome sky color and the darker color sky is the only way to make them look correct. (Deferred rendering toggle hotkey is ; )

note: using a color grading file to try and correct this instead of changing sky colors does not work, shadows will always have an annoying brightness to them compared to just turning Biome sky colors black.