The third story in Junji Ito's collection Deserter is called "Where the Sandman Lives," and I don't understand the ending. I'm going to try to work through it in this post. Full spoilers for this short manga. Incidental, and I think minor, spoilers for "The Face Thief" in the same volume.
Yuji is a writer born with no internal organs and a twisted self that lives in his dreams. Lately his dream self has been trying to take over his real body, flipping it inside-out in the process. That's because both Yuji and dream-Yuji want to get with Mari, his female friend. Mari stays overnight and tries to help Yuji resist dream-Yuji, but it's no good and she's sucked into his body. Presumably she enters dream-Yuji's dream world along with Yuji, since dream-Yuji escapes Yuji by flipping their body inside out. The police come looking for Mari and dream-Yuji informs them that "she's part of my viscera now," continuing it's no good to look for her, because "She's... gone on a trip in my dream world."
From Mari's perspective, this ending is kind of weird. The "real ending" for Mari is two pages earlier, when she decides to stay with Yuji and join him, trapped on the inside of dream-Yuji's body in the dream world. It's a gruesome and tragic end, but also a clear positive conclusion to their love story. And this scene with the cops doesn't change either Mari's or Yuji's situation in the slightest. So why continue on these extra pages? Just to give the story a more downer feeling? I don't think so.
Now that I think of it, this seems like a sad, shaggy-dog ending from dream-Yuji's perspective. It seems reasonable to guess that while dream-Yuji lived in Yuji's dream, Yuji's world was also Yuji's dream world. So, Mari started out as a person that he could connect with only in his dreams. He tries to enter that world so that he can connect with her, and he succeeds in entering — but in doing so, traps her in his dream world again. And what may be worse, he loses her because she's so in love with "the other him," because she cares about Yuji. We don't get to see a lot of dream-Yuji in this manga, these two ending pages are all we get, so these pages matter a lot to how we understand his perspective on what happened. In this reading, the ending emphasizes and clarifies dream-Yuji's lot: He chased his dream woman and lost her in his dreams.
That would match better with my read of "The Face Thief" (the second story in the collection). "The Face Thief" shows sympathy for the 'monster' Kamei, who is not inhumanly terrible and in fact seems pretty human in her needs and, metaphorically, her tactics. So, if this was a sympathy-for-the-monster ending, that would scan.
So I think that's right. Those two final pages are there to give us dream-Yuji's perspective and re-frame the story, which otherwise is Mari and Yuji's story.