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Perhaps, their Heartless all got killed within the same amount of time? Weird that they would all be together, but it seems to be a possible reason.
Otherwise, I got nothing.
Oh, so it's explained in this game how some lost their hearts?They all return to the places where they lost their hearts once both their Nobody and Heartless are defeated. Time has nothing to do with it.
I'm not far enough in the game to explain what Lea was doing in Ansem's lab and I'm not even sure if I will find out. But he was doing something there to have him lose his heart, even if it was a different time.
Oh, so it's explained in this game how some lost their hearts?
I'm guessing they all returned at the same time 'cause their hearts was a part of the Kingdom Hearts Xemnas had created. Once that Kingdom Hearts was destroyed all the hearts was released and found their way back to their nobodies.