Haven't CM Taloz spoken with you? It's technically not possible, that you got just half of the rewards for this achievement. If it was not working, you wouldn't get anything. Maybe you had full bag and it fell down, you didn't notice, maybe it went to another bag in your inventory and you later threw it away.
i checked every bag manually, then i put everything i have on me, and used items market > my offers > sell and i typed yellow box in the search bar, and it wasnt in my inventory, and yes i was talking with him, but i was still hoping theres hope...
my last login was at the end of 2017, i had a lot of space in my bags, i got teleported to my hometown after logging in so if its the case, and it fell on the ground, i would consider it a bug cause thats not my fault i got teleported, and i have no means of retriving the item if it fell on the ground, and im gonna say it again, i had lots of space in my inventory...
Anyways, so i didnt get it, and at this point im guessing im never gonna get it, but maybe u could check if theres actually a bug or something, cause maybe someone will come back and will be greeted by same surprise as i was
i feel like no one is considering it might be true that i didnt get it, and no one will ever check it, but as i said, since my last login was in 2017 i got some stuff moved around, few messages of changes, even my cloned scyther doesnt count as a catch in the new pokedex, my market locker was over capped, few items got deprecated, so i assumed it has something to do with it