Board Thread:Manga/@comment-27441457-20171108210024/@comment-27441457-20171113001611

The Lost Girls advertisement doesn't say it happened in the manga. It says "Annie's story is the clear winner here, since [. . .] happened in the story after all". To me, also considering Jean's reaction, this looks like she's spoiling her own story and is referencing something that "happened in the story" rather than suggesting it's canon to Isayama's manga.

No Regrets is more or less considered canon since Isayama has shown it his approval in interviews, his free time artwork, and brief refereces in RtS as you mentioned. So far, Lost Girls hasn't gotten that treatment yet.

I can see Lost Girls being declared canon more definitively in Isayama's manga in the future though. There's a difference between not being acknowledged as canon and openly being acknowledged as non-canon.