I am having this very strange visual glitch with my game. It only occurs using shaders, including internal shaders. I am using 1.12.2, Forge version 14.23.5.2838 and Optifine version 1.12.2 HD_U_E3 (after much trial and error finding a version that did not crash my game). The game doesn't crash, and actually runs very well, but I have no idea what could be causing it
Issue was resolved. Turns out Stellar Skies was the cause of the issue, likely conflicting with another mod