This happened well over a decade ago. No, it had nothing to do with the resets. If there had been resetting of our timeline that long ago, I wouldn't have remembered anyway, like any other monster.

Not their fault. Not Flowey's. Only his.

Yes. I'm not all gone. Look on the bright side, I'm able to think clearly, to stay in touch with you three, and so none of you have completely lost me.

For what it's worth. Frisk and their brothers (in spirit) still have the Gaster they met. Let that be enough.