Only a visual issue which might have been here since the beginning but most of the scaffolding blocks look like this:

Some scaffolding blocks like the stairs and slabs don't have this.
I have checked if optifine had something to do with it but it is just part of the mod.

That's because those aren't waterloggable, whereas stairs/slabs are.

Done some digging, as of 1.15+, Forge added the Block.shouldDisplayFluidOverlay() hook for controlling this:
MinecraftForge/MinecraftForge#6493
Though scaffolds probably should be waterloggable?

Interestingly, glass blocks are not waterloggable, and do not suffer from this issue.

This is definitely something wonky with rendering, but I'm not quite sure what. I'm open to any input

Scaffolding is now waterloggable :D
Along with various other blocks