Did you wait for your troops to arrive at the quest? The troops don't travel instantly from your city to the island quest so it may be a reason why the quest still said you didn't send the troops.
If you waited and they disappeared, clear your cache and cookies. If they don't reappear once you've done this and you're sure that you did send these units but they disappeared, contact the
support and explain your problem, we can't really help you here on the forum ;-)