Hi,
I have the same issue, it is defiantly doing it on movie libraries, it might be on TV but that’s harder to follow. I only noticed it as old movies appear as newly added lists but they were not new nor had I added a newer version.
Here’s what I know so far:
Setup, 2 Synology NAS boxes, one mounts some films from the other via CIFS.
All films on the local film folder are present and correct
However, at least one film vanishes from PLEX from the film folder via the CIFS mount.
You can see the films folder and the films files fine via file browsing
I SSH’ed into both NAS’s and all permissions are correct, simple and identical to the other film folders
I used sudo -u plex to check the plex user on the master synology box could see the film folder that is missing from Plex server, it can so the plex user can see the folder/film files fine from command line.
This is when it gets very very strange.
If you move/delete the missing film and it’s folder, scan the library from the GUI, another film vanishes, and so on, if you work out which film vanished, move it’s folder from that location, scan, another random film vanishes. It can be more than 1 film as well, so you could be missing 2 films, if you move them out of the folder, two more will randomly vanish.
If you install plex on the CIFS shared server, it finds all films no problem, so local folders/files are fine. So this only happens if you are scanning a folder that is mounted via CIFS/SMB, local folders are fine.
More investigation, can PLEX see it from the GUI no it cannot!
I know which film is missing from Plex Server, I add a new library via plex giu, I browses to the folder where the film is missing, and the folder is missing from the plex gui.
As above, if you remove the folder from that directory another film folder vanishes, add a new library and now the new missing film folder is not visible to the plex gui.
I rolled back the plex version so far all the way to PlexMediaServer-1.23.1.4602 and it is still doing it. I’ve changed the library agent still doing it, as once a film/folder is missing the Plex GUI cannot see that folder at all!
EDIT: if you move a missing folder out of the path, scan, another movie vanishes, you can then put the first back scan and it appears so it’s not on an ignore list. Doing my head in this one!