I had the same problem. After turning off all the download options in Settings it finally cleared the downloaded episodes out. Just didn’t happen right away. Let me know if that doesn’t work.
Discussion
I guess the iPhone settings can just not be trusted. After deleting the app the whole data-usage did not change so that value was already showing the right amount while the specific Fountain-App row still showed 10 GB which wasn‘t really on the mobile anymore at that point. 👍