Ok just changed all mods to 1.12.2 and tried it again, same problem. When OptiFine is in the mods folder I can't see the outline of the LT blocks

forge version: 14-23-4-2759

I tested it out and I couldn't replicate it. When you say outline, you mean like this, right?

Or maybe the preview?

I'm also using the same mod versions as you, so maybe an setting on Optifine is disabling it?

ok yeah, I did mean the preview

here's without OptiFine in the mods folder (or shaders disabled)

and with Optifine active (shaders enabled)

Now when I turn shaders off in the shaders menu, the preview is back

shader I use: Wisdom-Shaders-V3.2-Release-Build3230

Yeah that's a shader specific problem. I seen the preview work on some shaders, but never 100%.

Alright, do you maybe have a list of shaders (that might work) that I could try out?

BSL, Seus, Slidur, actually most shaders should work. Unfortunately there is nothing I can do about it. It's an issue of the shader.