I can see why someone who knew how to pace a novel might not know how to pace a short story, or vice versa. But when someone's dialog gets clunky and weird in a novel, and it was crisp and reasonable in multiple short stories, I'm confused. Similarly, I can see where one might think high fantasy required or at least benefitted from a different kind of description than urban fantasy or magical realism -- but why would the pacing go south during the switch? I can see where someone might overthink vocabulary and sentence structure when switching from adult to YA, but where would the total lack of character development come in?
It's a little daunting as a reader, wondering where someone is going to just suddenly stop doing something next.