This does not seem to be model based and is instead an issue with the rendering engine. Not a fan of this bug.

Damn, I can't actually reproduce this one.

Screenshots?

Doesn't look nearly as opaque.

Oh, it seems that I can reproduce it in vanilla Minecraft, but not with Forge.

Wait, what? The rewritten version of RandomPatches seems to fix this issue somehow, but I never implemented anything of the sort... 😕

Oh, I'm stupid. I just realised that I did and forgot about it. I was testing a line of code that I thought might fix it, but I forgot that I left it in. Needless to say, given how simple the fix is, I'm surprised Mojang hasn't fixed it yet. 😛