To reproduce this:

1 - Go to a cave
2 - Set time to "Noon"
3 - Set time to "Midnight"

Caves are darker at nights and brighter at day time.

It is not a bug. It is intended to work that way.

The brightness setting inside the caves "steals" outside light. This allows the caves to be "illuminated", and at the same time, give notion of the time of day in which you are. Caves are never brighter than the outside.

If they were always lit, they could be brighter than the night around them. If they stayed dark, they would be practically black all the time, and everyone complains about how dark the caves are already, so ...

This happens because the difference in brightness between night and day is much greater than in the vanilla game, due to the high dynamic range.

If you want similar cave lighting between night and day in caves, you can minimize the brightness inside the caves in the options, but they will be practically black all the time (as they really would be)

But it could work differently, it's true. It is an inheritance from the original lighting design.

I will contemplate the possibility of making the caves with constant lighting.