A new movie added is already marked as played because the trailer (for that movie) was shown months before it was added as a cinematic trailer before a movie.
For example:
Watching the movie Avengers: Infinity War (on August 2018), plex plays 2 trailers (as set on the Plex Player which is my Samsung Smart TV) before the movie, the trailers were ANT-MAN and the WASP and Hotel Transylvania 3 and then the custom pre-roll I made then the movie. Both ANT-MAN and the WASP and Hotel Transylvania 3 are now released and added to Plex but both movies are also marked as played.
I recently marked as unplayed the movie Hotel Transylvania 3 but Sunday night, we watched Unfriended Dark Web and the trailer for Hotel Transylvania 3 played again before the movie and wouldn’t you know it. Plex once again marked that movie as played even though we haven’t even watched that yet. This happens to all the movie that their trailer played. Why is that happening?