I'm actually with manga-only readers on this. You see, these two chapters worked in the WN because we already got tons of side chapters before this, to us this was just another side chapter. A good one actually, because it added some mystery to this character's real identity, and it will serve as the first major plot twist in the WN.
However, the manga already ignored all the other side stories, so I don't know why they didn't just made this manga as linear as possible. It really broke the flow of the manga.
I myself am really happy to see this girl since I read the WN from the start, but I understand the frustration of manga-only readers