I can't 100% confirm that it was after a loss (the party leader backed us out after every match so I didn't see my drops all the time), but I did notice I had obtained amduat ink after a loss and I don't think it was there before that. That said, other than that circumstance I don't think I've ever heard of or seen a shader drop on a loss so it was probably for a win I had and I just didn't notice the drop.