Do you see the little portion of map over to the right, indicating you stepped in and weren't able to go any further (because of some pillars)? That's not a real dead end. The passage is in the ceiling, with a midair morph. Oddly enough not too many people mentioned getting stuck there (probably just because of the way the room just dumps you into it when you go through the door and it doesnt seem logical that it'd be a dead end), but it is a rather deceptive area.
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