No, that's how it's always been! See, my Grandpa was only one Avatar. Once he died, his spirit was reborn as the next Avatar, and she can access all his memories and wisdom. The spirit of the Avatar is always reborn anew, in the Avatar cycle. They're the ones who are supposed to keep the four nations balanced and in harmony.

And to deal with Spirit World things. [ Ikki shrugs. ] Bending isn't any weaker - if anything, I think people learn how to use it better as years go on, y'know? Like, when Grandpa was alive his friend, my Aunt Toph, figured out how to bend metal. No other earthbender had known how to do that, before her, and now a looooooooot of them do.