Minecraft 1.16.1
Forge 32.0.108
Optifine U G2
Optiforge 0.1.9
Modpack Valhesia 3 3.0.18

Shadows will follow the player in specific areas. They are related to optiforge and not the shaders themselves. Turning off optiforge fixes the issues. Switching between different shaders (Sildur's, SEUS PTGI, BSL) does not help and shadows persist. Turning off shadows (Sildurs), increasing shadow distance (from 120 to 240, SEUS), appear to fix the issue.

Video taken with Sildurs Extreme VL, note the bluish shadow,
https://i.imgur.com/0h7LQ8h.mp4