In 1.12, lightmap updates can cause flickering at high gamma. This feature patches net.minecraft.client.renderer.texture.DynamicTexture and Lightmap updates to keep brightness perfectly stable.
Here’s a for a Minecraft mod or cheat client utility named "Fullbright 1.12" (for Java Edition 1.12.x): fullbright 1.12
Illuminating Your World: A Complete Guide to Fullbright 1.12 fullbright 1.12
Fulbright effectively overrides this, setting the gamma value to an extreme high (often exceeding the standard GUI limits). The result is a world where there is no darkness. Caves are as bright as the surface, and the night sky is barely distinguishable from day. fullbright 1.12