Maybe you quit and the entry will not be refunded.. or you got connectivity issue cause it happend to me on the last fight with Guillotine, after the fight was won I got disconnected from game and when I came back the entry was used but it did not count towards the objectives, luckily it was only in Master difficulty.
@DJMNH Our Support team should definitely be able to help you out here. Please continue to liaise with them. Thank you for your cooperation!