I beat the Onyxia bounty (on both heroic and normal), and then put in a ticket after not receiving a reward. I was told to also complete heroic Undead Onyxia, which I did, and I still did not receive my reward.
My team did have three dragons in it in every attempt.
edit : is task 5 or task 9 ?
edit2: are you using natural dragons ? a trasfromed Kazakus will not count as a Dragon i think.
edit3:
task 5 require normal Onyxia and 3 Dragons
task 9 require heroic Undead Onyxia and 4 Dragons
It’s task 5, I used Brightwing, Alexstrasza and Sinestra.
Hm weird. It completed just fine for me.
Just in case it’s relevant: [Mercs] C'Thun task 5 not completed on Heroic