Sometimes I find super dark & odd shadows on Overworld mountain sides / cliffs ... and not only does breaking one block in that area immediately remove the entire shadow ... but those spots always lead to caves. Which immediately led me to this mod.

Removing BetterCaves fixes the problem. =(

MC: 1.12.2
FORGE: 2847
Optifine: F5
BetterCaves: 2.0.4
Quark: 179
JEI: 291
VanillaFix: 150
Rustic: 1.1.4
ChunkAnimator: 1.12-1.2
Ceramics: 1.3.7
ElectroBlobsWizardry: 4.2.11
Baubles: 1.5.2
No Shaders or Resource packs are used

BetterCaves on its own doesn't cause any such problem, so it's probably a compatibiltiy issue. Can you post screenshots?

Closing this due to inactivity. Pretty sure it's just the 1.12 lighting engine being shitty - I suggest getting a mod like Phosphor, and it will probably fix the issue.