Destiny Stinger vs Altered Creation GX

This has happened a few times now, so I think there is a bug with the way Altered Creation GX calculates knock-outs and prize cards to take. For clarity, the text for this reads:

"For the rest of this game, your Pokemon's attacks do 30 more damage to your opponent's Active Pokemon (before applying Weakness and Resistance). If this Pokemon has at least 1 extra [water] Energy attached to it (in addition to the attack's cost), when your opponent's Active Pokemon is Knocked Out by damage from those attacks, take 1 more Prize card.

When I use Destiny Stinger (my Beedrill's attack) successfully, it KOs me and my opponent's Pokemon. I gain as many prize cards as the opposing Pokemon is worth, and my opponent gains one for the KO of my Beedrill. However, if my opponent has previuosly used Altered Creation GX (Arceus&Dialga&Palkia GX's attack) to its full ability, it awards my opponent two prize cards for the self-KO of Beedrill, instead of just one.

This is wrong for two reasons. Firstly, my Beedrill was not knocked out by an attack from my opponent, as it was a self-KO. And secondly, it was not KOed by damage at all - it was an effect of an attack. I really can't see it what way my opponent is entitled to take an additional prize card in this circumstance. There have been no stadium cards or other effects that change the amount of prize cards taken, so I think this is a bug in the way Altered Creation GX calculates prizes.

Does anyone have any thoughts on this?