@Vincentius: Even so I feel the author may have cut this short earlier than planned if only slightly. Like right after fatty is defeated they clean up the city, save the fake little sister, then for some reason that saves the girl even though normally saving a trapped person requires them to reject the Amisogi. Instead saving her wasn't at all related to her and was instead about doing the "save the sister" quest. They add exposition at the end but it's strange for the explanation to be shoved into the epilogue like that.
Overall it really makes no sense that the situation was resolved like that given how past cases worked. So I'm guessing this was origionally meant to go on longer and after killing fatty they were meant to save the sister and either before or after make the girl accept the world is fake and return with them to the real world. The explanation of her IRL little sister dying was either not the origional idea or it was meant to be explained during part 2 of the isekai arc.
But readers were getting sick of the Isekai genera shift so the author decided to axe the arc early after at least defeating the "mid boss" who was changed into the final boss.
Again I'm not saying it ended too early as it actually dragged more than it should have. But the WAY it ended felt unnatural and didn't seem to follow the established rules of saving someone from an Amisogi. Every other time they had to confront their inner demons unless the MC straight up destroyed it directly. But since neither happened something feels off.