The app removes episodes from the currently active playlist once they're completed, If the media files haven't been deleted yet....you can add them back to the default system playlists from the episode screen of each particular feed/podcast on the "Feeds" tab
Press the context menu icon (Hamburger Icon Top Right of Episode Screen) to flag all episodes new(Blue)/in progress(Green) or press the context menu icon of each particular episode (Hamburger Icon Right of Episode title) to flag new(Blue)/in progress(Green)
Another option is to use the Multi-Select feature to flag all or selected episodes as new/done, delete media files or add episodes to a user/dynamic playlists> to enable "Long Press" on a particular episode from the episode screen & then press on selected episodes> press on the icons on top of the screen to enable selected actions
Press the Playlist icon to add new episodes to a playlist (User/Dynamic)
Press Flag New icon
Press Flag Done icon
Press the delete icon
Press the Pinned icon to deleted pinned episodes
Currently it's not possible keep completed episodes on the current active playlist automatically