You seem to forget about when EVERYONE forgot about sora due to namine's meddling. I just try not to think about little plot discrepencies like memory unless they directly contradict canon. Also I've forgotten, like, months at a time so if it can happen to me then maybe cartoon dog done goof'd.
God, I freaking knew someone was going to bring up COM.
Everyone forgot because it was part of the plot. So unless NaminÃ© was conveniently unable to restore Dolan and Gooby's memories about the Keyblade, Yen Sid's tower and such, then it's a plot hole.