Minecraft version: 1.12.2
MoreOverlays version: 1.15.1
Twilight Forest version: 3.10.1013
Forge version: 14.23.5.2854

When you're in the Twilight Forest and enable the lighting overlay (F7), the lines almost never appear anywhere regardless of light level.

Except for in the three miniboss biomes surrounding the Snow Queen, Hydra and Ur-Ghast bosses (i.e. Snowy Forest, Twilight Swamp, and Dark Forest respectively). In here the lighting overlay appears to wildly flicker randomly although adhering to the correct spawning colours. Progression in the mod (related to your access to these areas) does not affect the flickering. I'm still assuming however, that the wild, random flickering might be related to the particle effects within these biomes, which appear if you didn't have defeated the appropriate bosses yet.