You read the au. Sayako lost her memory due to drinking a memory loss potion when Tekara captured her in S2, everyone freaks out and tries to restore her memory again. They try everything.
being back in the TAHS was the worst already. oh, but...
Green liquid. Looked like kool-aid. tasted sweet. Like kool-aid. had fizz. like soda. burned a little. like-... oh no. But don't worry! just took her memory of everyone she loves and cares about!... okay, maybe worry...
The only way to QUICKLY restore her memory, is to give her a very delicate and hard to make potion. So they save that for a last resort.
Good thing Sayako has some good and dedicated friends, huh?