Custom items in an ItemGroup are only usually checked for when working with Armor and Weapons (and sometimes clothing). This prevents custom items from being randomly generated (or properly generated like Clothing in shop shelves) or added via console commands.

Instances of missing custom item checks:

1. ItemBuilder.CreateRandomBook()
2. ItemBuilder.CreateRandomReligiousItem()
3. ItemBuilder.CreateRandomGem()
4. ItemBuilder.CreateRandomJewelry()
5. ItemBuilder.CreateRandomDrug()
6. ItemBuilder.CreateRandomIngredient()
7. ItemBuilder.CreateRandomPotion() and CreateRandomRecipe() - Unsure if modded potion recipes are checked for
8. DaggerfallLoot.StockShopShelf() - Custom items are added, but only Weapons and Armor will be initialized with dyeColor, CurrentVariant and PlayerTextureArchive
9. DefaultCommands.AddAllEquip() - Only Weapons and Armor ItemGroups are checked for custom items in the group

With custom item check:

Without custom item check: