Lost thumbnails when indexing: Synology 7.2

Where did you put your Linux installation files?

Did you put them into the Plex shared folder and then let DSM 7 Plex import them
-OR-
Did you place them directly in PlexMediaServer?

Importing PMS from other platforms is done through the Plex shared folder.

If you still have your original Linux server tar/zip file, the best next step is to redo it.