Brightness cannot be altered outside of the vanilla capabilities when played with Alex's Caves.

With Better Brightness Slider at -50% without Alex's Caves

With Better Bightness Slider at -50% with Alex's Caves

Brightness cannot be altered outside of the vanilla capabilities when played with Alex's Caves.

With Better Brightness Slider at -50% without Alex's Caves

With Better Bightness Slider at -50% with Alex's Caves

I am not apart of this mod, but I have got it working with Alex's Caves. You may need to change versions for Alex's caves or make sure ALL of its mods and core mods are updated. This worked for me when I added it to a preexisting pack I made.

The reason for this is the Vanilla Darkness effect. When Mojang added it, they made sure that you can't have brightness lower as 0%, probably because it looks weird with the effect. In my mod I have just re-enabled that feature with a Mixin into the lightning engine, but when other mods change the lightning engine (like Alex's Caves does), it doesn't work anymore. I'm not sure if there is a good fix for this, but I'll look into it in the future.