I am using the latest Build of Gamestage and Tinkerstage, and have successfully staged my Tinkertools.

So if Player not have the requirement Stage, then the Item will dropped on the ground. That works.

But if you hold that Item in your Offhand, then you can use it like a normal Item.

So how you can reproduce?

This is my Script:

mods.TinkerStages.addToolTypeStage("end", "tconstruct:cleaver");
mods.ItemStages.addItemStage("end", tconstruct:cleaver);

Try that with Tinkers Construct, and equip Weapon in your Offhand.