In vanilla, the nether set a brightness value all across the board. Something is squashing the brightness that was hard set in vanilla in the new rendering pipeline.

Currently, the nether appears as if it's nighttime in the Overworld.

We'll have to investigate how the nether was lit in the vanilla system, then create a hack, or some sort of patch to set a per-world brightness render setting that will raise it back to an expected level.

A similar issue occurs with Night Vision potions

Night vision potions have been fixed, Nether brightness is also fixed.