Because me and somebody else (Fehlende item-tags der episoden im feed) stumbled over this “issue”, I write a post in this community forum with the hope that it helps other people (and/or my future self).
When the polylang plugin is installed (in multi-language site settings), episodes will not show up in the feed(s) until language capabilities are configured in the polylang settings for the episodes Post Type. Then, either default to a language or set individually in the episodes overview.