EDIT - I found the answer. Took about 10 minutes, but the materials showed up in my mailbox.
I created a couple of work orders for inscription treatises, but for some reason the game made them guild orders rather than public orders. No one in my guild can make these, so I cancelled the orders and went to recreate them using the public option, but I no longer have the materials. The orders show up on the “My Orders” tab as cancelled, but I can’t reclaim the materials.
Are the materials - including my Artisan’s Mettle - gone for good, or will I get them back when the order expires (in 48 hours)?