I think the author wanted to introduce new characters but later just conveniently added in the amnesia so that the MC wouldn't be caught by Lucia's ability. Or maybe it's just that the arc was longer in the LN and got cut short here.

Tight story that didn't drag itself out, wrapped everything up and ended well. The only gripe I have is why would you call it "Side Story" and not "Epilogue"?! I almost missed it because of that.