I have seen that recently and I'm pretty sure it means that the episode was in the feed at one time, hence in the media library now, and now the feed no longer has that episode. I saw this with an audiobook podcast and it was a temporary episode saying that I caught up with the author. Once the author did the next episode, there was no reason to have that temporary episode anymore, but it was still in the library and said "Not in Feed". I just had to delete it from the library.
