Qbittorrent's official flatpak, despite being the latest release, is built with libtorrent-rasterbar 1.something. Qbt i2p support is dependent on libtorrent-rasterbar 2.0 or greater.
Just putting this here because I experienced the pitfall of wondering why the hell my latest version qbittorrent didn't work with i2p earlier. Thanks to the people in #i2p irc who helped me troubleshoot this.