My friends and I were trying out some Shaders to use, just to make the game look more detailed and (somewhat) realistic. Due to the mods we're using, we need to stick with 1.12.2. While using them, we've noticed that, in each one of the Shaders, the lighting is incorrect on the inner corners of the blocks (see screenshots, below). This reduces depth from the blocks, and makes it look less realistic. This is something that is done in normal Minecraft, but for some reason we weren't able to make it happen with the Shaders.
After that, we did a little more digging. We tried using older versions of the game, and found that the issue actually came from Optifine, way back in the days of 1.9.4. The lighting is just fine using Optifine version HD Ultra B6, but move forward one version of Optifine (in the same version of Minercraft!) to HD Ultra D7, and the corner lighting is gone. We tried 1.10 and 1.11, but found that the issue persisted. We also tried it on a different computer. One that didn't have Minecraft, didn't have Forge installed, didn't have optifine installed before, nothing. Really good computer too. But even then the issues still persisted (for reference, the two computers had the following graphics cards: Nvidia Geforce GTX 970M, purchased in 2015, and an Nvidia Geforce RTX 2060 Super, brand new). We tried using a number of other Shaders from other creators, but no matter which ones we chose, the issue remained. We have therefore determined this to be an issue with Optifine and Optifine alone.
But the thing is, we haven't been able to figure out what changed to cause this. We looked at the changelog for Ultra D7, but couldn't determine what change may have caused this to happen. We've scoured the video settings of Optifine and the internet and have come back empty-handed. We haven't even found many people who have complained about the issue at all, let alone any solutions. We don't know if it's something that a lot of people have overlooked, or what, but after hours of searching, we've given up and decided to ask for help.
Does anyone know what's going on here? It's driving us insane.