That is true. He didn't wish to have his memories changed back to the way they were.
REGISTER TO REMOVE ADS |
Riku lost his memories, when he became Ansem, but then he retrieved them back.
I think some may have disappeared in Kingdom Hearts: Chain of Memories. I don't know how, but something must have happened.
Or something happened when Sora was in "Deep Slumber" for a year. That could be possibly explained in Kingdom Hearts: 358/2 Days.
I have another guess, too. When Sora defeated Xemnas in the "Final Battle" he could have destroyed the whole Organization 13.
Those are my guesses ^__^