Items only appeared in collections if they were in your actual inventory at any time since Warmind launched, so if you only had it on your gear, and not in your actual shader inventory, that's why it's not there. It's possible Tess might sell it someday, as she occasionally rotates old items into her weekly items for sale, but if you dismantle an item with the shader, you [i]should[/i] get the shader back. If I were at home right now, I would try it myself to make sure, but if you have an item you're willing to sacrifice, especially one you can get back from your collection, that's your most likely bet.