by Kwaine
3 things made me think that it is the way it works.1 - In the rule book page 19 :
Mice keep their story achievements and abilities earned from previous chapters.
2 - Story book page 6 :
The party receives the Miz Maggie Ally story achievement token. Advance the chapter end marker 1 page on the chapter track.
3 - In the story book page 5 :
Helping Miz Maggie will get players the Miz Maggie story achievement, which could help in later chapters.
So I asumed that this achievement effect would be there for any subsequent chapters, but this is VERY poweful, so maybe this is not true. I didn't finish a lot of chapters yet, so maybe there is one where Miz Maggie help you if you have her achievement or something like that.