this is off topic, but I'll tell you about my own experience. I had the same problem 2 days ago when I added some comic books using Calibre. the first book in 12 books series didn't appear even though I chose 'showing items by Most Recent First'. however, when I searched the title it appeared first on the result list.
Well, I just ignored that and went on with my reading. after that it appeared on the list as if nothing happened
don't worry, kindle is crazy at times
and nice to meet you.