Somewhere in 1.17's development, the brightness of sky light has been reduced. Before, it used to be #FAFAFA, but now in modern versions it is around #E0E0E0. I did a little bit of digging in the code but I could not find any reason why the sky light got darker, but I suspect that it is something to do with the addition of core shaders.

Left: Before 1.17, Right: After 1.17

How to test this yourself

1. Set your brightness level to moody
2. Create a new superflat world with the "Snowy Kingdom" preset
3. Set the "doDaylightCycle" gamerule to false and set the time to noon
4. The snow texture contains the color #FFFFFF both with and without the programmer art resource pack. Pick the color of the area of such color. If you're playing on 1.16.5 or older, you'll get #FAFAFA. If you're playing on 1.17 or newer, you'll get either #DFDFDF or #E0E0E0.

The fix

Minecraft's lightmap UVs seem to not reach their full possible range. A quick and dirty solution to this is to simply replace line 14 in "assets/minecraft/shaders/include/light.glsl" with this:

return texture(lightMap, clamp(uv / vec2(255.0, 247.0), vec2(0.5 / 16.0), vec2(15.5 / 16.0)));