You will need to rescan and you may need to “Plex Dance” the whole series and that usually resets the watched status.
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When you change anything to fix a match it is a good idea to “Plex Dance” the files involved:
1 - Move the media to where Plex cannot see it
2 - Refresh the library
3 - Empty Trash
4 - Clean bundles
5 - Move the series back
6 - Refresh the library
Perform all steps in order and allow each one to finish before starting the next.
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