Correct me if I'm wrong, but:
`case.SMITHING` is missing here?
https://hub.spigotmc.org/stash/projects/SPIGOT/repos/craftbukkit/browse/src/main/java/org/bukkit/craftbukkit/inventory/CraftContainer.java#157

When opening a new InventoryType.SMITHING to player it opens a 9x3 generic inventory named "Upgrade Gear"
Opening an InventoryType.SMITHING_NEW does however work as expected.

Sorry I don't have code to test, I was helping someone with something in Skript when I noticed this issue.