Try starting your app via the command line with the flag
The functionality is not broken, it is intentionally denied access. Flatpak apps are extremely secure, often to the point of being nearly unusable. This security comes from running each app in its own sasndbox.
By default, This sandbox is cut off from almost access to the outside system. For more detail, and a few more methods of circumventing this behavior, see [here](Flatpak apps do not detect headphone microphone).
Honestly, this solution is a bit of a hail mary. If it doesn't work, try starting with the verbose flag. If you see anything about denied permissions for pulseaudio in the output, then it means that whoever packaged the app for flatpak did not build it properly.
In this case, access can only achieved by running with
sudo, and that should be avoided with any app that sees the outside world, such as networking apps like yours. The better option would be to download via a different channel.