oooh, encountered this exact thing very briefly a few days ago when i did that with a premium comic. you probably already tried this, but i logged out then back in on the app and the episode appeared just fine afterwards.
also in case it's helpful to know, i used that new(? i think it's new anyways, i hadn't seen it before) one-click option which pays for all the episodes for you as they come out. i hadn't seen it before that point and assumed it was just a a tiny glitch from it being a new feature.