I have the bottle of Chateau de Vinegar and the blasting soda.
If I look in the Creatable tab in the Item Manager, Unstable Fulminate isn't listed. This is regardless of whether I have require-in-a-boxes selected.
Same goes for the Cookable tab (since it is cooked) and the Mixable tab (wouldn't expect it to show here though).
It's showing up in my discoveries tab in the relay browser as cookable so there's no missing requirements or anything as it cooks as soon as I click the [cook] link there.
KoLmafia v17.12 r18370