I use fresh minecraft with:
1.20.1 embeddium+oculus, cull leaves 3.2.0
1.21.1 sodium+iris, cull leaves 4.0.1
Both version has 9.1 better leaves

I see a big difference in the display of foliage and 1.20.1 has less darkened texture

First screenshot - 1.20.1, second screenshot - 1.21.1

This issue is exclusive to Complementary Shaders.
I got the following suggestions by EminGT (author of Complementary):

You just need to go to Shader Settings and change the profile to "Very High" or "Ultra".
Or instead get more specific and go Shader Settings -> Performance Settings -> Real-Time Shadows -> set it to "High" or above