This is something that has been confirmed as not working yet by Plex, somewhere in this thread. I can’t find any workaround at this point & it is very annoying as if I want to use PIP I need to stream the content (not great for time away from home and very expensive mobile broadband).
https://forums.plex.tv/t/sync-is-dead-almost-long-live-downloads-part-2
(tried to find the specific post, however couldn’t as the tread is rather long due to the many issues).
PS - Here is a feature request post for the very same I popped in a little while back, let’s hope it gets some attention.
https://forums.plex.tv/t/feature-request-picture-in-picture-with-downloaded-content
