I'm having the same issue as described in #41 I'm afraid. Seems this bug is still present, or has once again reared its head.
V-Tweaks 2.0.4, Forge 14.23.5.2847, and Minecraft 1.12.2.
The only difference is, instead of a purple-and-black cube, it was a pure purple cube (though that is likely due to me using a texture pack which may alter the placeholder texture).