Like the title says, for some reason when I have shaders activated and this mod on and running, the outlines don't ever show up

Shaders and OptiFine are not supported, you can read about why here

That's weird cause it has been Optifine and this mod has been working perfectly fine for me in 1.16-1.18. Its only when I added a shader that it stopped working

It's specifically shaders that cause the issue, but optifine in general is closed source and Mikey has not wanted to deal with the pain that is OptiFine bugfixing