open the web app https://app.plex.tv/desktop
go to Playlists on the left side
scroll to view the icon of the affected playlist
does it have a small ‘cog wheel’ icon on top of its poster?
If it does, it means this is a ‘smart’ playlist, which was created with some filters.
A smart playlist does store the original filter rules, which were used to create it.
The most basic example would be a smart playlist which is consisting of ‘unplayed’ items.
Afterwards, as soon as those items are played, they get removed from the smart playlist.
