I noticed this the last few times I traded a super rare dragon (Black Knight, Scorpion). The trade pop up shows gems and a mystery box, but when I collect the trade, I only get the gems. Has something changed with the trading? Is this because I only level up my dragons to L4 to trade? Seems unfair we haven't been notified of this if it's changed.
I used to get the same problem a while back, but it was connectivity related. I would collect a trade missing the extra item, but then the game would sync and it would show I hadn't collected the trade. I would then collect it again with the right number of items returned.