Arceus & Dialga & Palkia GX Altered Creations vs "Prevent All Effects Of Attacks"

It does say "Prevent all effects of attacks" so while the effect is on the player, TCG Live seems to be creating an exception to that effect on the player for this one Pokemon. But like you said, ptcgl might be getting it wrong too.

Upon further review, as per PTCGL, Hoopa [SLG] and/or Snorlax [LOR] don't have to be the defending Pokemon at that time the Altered Creations GX attack goes off. If either of the Pokemon are knocked out at any time later in the game after the Altered Creations GX has gone off, only 1 prize card is taken. It may be that they need to be on the board (active or benched) at the time of the attack or maybe they don't even have to be on the board, I'll try to test that out. PTCGL seems to treating the effect of the Altered Creations GX attack on ALL the opponents Pokemon and then exempting these Pokemon if they were on the board at the time of the effect (and possibly even if they weren't) due to their ability. Just wanted to give a more accurate picture of the exact mechanics.