Marking something as “watched” no longer creates a “played” entry for it, something actually has to be watched within the Video played threshold for it to register as actually played. This change came in server version 1.40.0.7998 → Plex Media Server - #612 by drzoidberg33 .
As a workaround you could play the previous episode and fast forward it all the way to the end. Plex will see this as played. Or you could just watch the new episode which will place it back on the continue watching hub again.
