Hello, I was play your mod and in the caves I was having light update issues along with quite a lot fps drops. I have Optifine installed but it's not the problem

Minecraft: 1.12.2
Betweenlands version: 3.3.7
Forge version: 14.23.2.2654
Optifine version: HD C9 Ultra

Here's some screenshots

This is usually caused by "Force threaded chunk rendering" in Forge's config. It's not really a bug, just a tradeoff for improved performance.