Watching another episode always works for me. I collect episodes of a Youtube series, but only watch every few episodes. When I watch a single episode, it keeps popping up on “Continue Watching”, and I have to remove it again.
Are there any “unplayed” episodes of the show? If they are all watched, then it will not offer to “Continue Watching” any episodes unless you are mid-episode. Which might be something worth trying: Start an episode, and stop about ~2 minutes in. A Continue Watching entry should show up now.
Stopping an episode mid-play caused it to suddenly populate the Continue Watching hub. Then after that finished it filled the next episode as expected.
Up until that, no amount of marking played/unplayed (even resetting the entire series and marking a few) would work.