Light and shadows are definitely not behaving normally with floodfill on.
Some information:
amd drivers
windows
minecraft 1.21

Video:
https://github.com/X0nk/Bliss-Shader/assets/64977575/3ad3f1bd-7be9-4565-8bb2-73995a80ee7f

Bliss config:
Bliss-Shader-main.zip.txt

mods list:
mods.txt

same as this issue over here

mb mb, but I also have flickering light

same as this issue over here