Keep in mind that Luna didn’t read all of the note before tearing it up—and for that matter, it’s unclear how much of the note she’d be able to read, since she can still barely wrap her head around magic. This means that even now that Xan is trying to communicate openly with Luna, it’s almost impossible for her to do so. Now, Xan has to face the unintended consequences of forcing Luna to forget—namely, that Luna is hurt and angry that Xan kept things from her and left with only a note.