Video Playlist Items disappearing

I create a playlist with 152 movies in it. I use the web interface, look through my movie collection and add movies to my playlist from there. I go to my playlist and they are all in there, atleast for now. It seems that over time, more and more movies drop out of the playlist. I had 152 movies in it, now its down to 109. Next week it will probably have much less. This is the 3rd time I created a large playlist only for it to keep happening. There is no way to get it back, and very frustrating to go back and forth figuring out what is missing. How to stop this from happening?
A thought would be, is there a way for Plex to put some sort of flag on my movies so when I am looking through them it would say if it is currently in a playlist or not? That would make it so much easier to identify what movies are missing while looking through my movie collection.

duplicate of Why does the items disappear in the playlists (using the web app)

closing