This is the code I'm using and it mostly works.

`#loader contenttweaker

import mods.contenttweaker.VanillaFactory;
import mods.contenttweaker.Block;
import mods.contenttweaker.Item;
import mods.contenttweaker.IItemRightClick;
import mods.contenttweaker.MaterialSystem;
import mods.contenttweaker.Material;
import mods.contenttweaker.ResourceLocation;
import mods.contenttweaker.CreativeTab;

var scrap_tab = VanillaFactory.createCreativeTab("scrapyard", item:minecraft:fireworks);
scrap_tab.register();

var ruby = VanillaFactory.createItem("scrap_ruby");
ruby.rarity = "rare";
ruby.creativeTab = scrap_tab;

ruby.register();`

crafttweaker.log

• Forge 14.23.4.2760
• base-1.12.2-3.10.0.jar
• blockdrops-1.12.2-1.4.0.jar
• ContentTweaker-1.12.2-4.8.0.jar
• CraftTweaker2-1.12-4.1.9.jar
• jei_1.12.2-4.12.0.216.jar
• modtweaker-4.0.13.jar
• MTLib-3.0.5.jar

Hope its something I've just missed.

Fixed by #143

Other tabs are ok