Cannot join netplay with Linux flatpak

I am on Debian GNU/Linux using the 1.9.1 Flatpak build

I can’t join any multiplayer games. I can host, but when I try to join a game by going into the netplay menu, the Retroarch window just disappears. The process is still running and doing apparently nothing, and I have to killdashnine it (SIGTERM doesn’t seem to work).

If I first run the game I’m trying to join and then go into the netplay lobby and try to join there, the game freezes for a while and eventually I see this in the logs: (IP address redacted, but it is correct)

[INFO] [Netplay]: Connecting to (direct)
[INFO] [Netplay]: Connecting to netplay host
[ERROR] Failed to set up netplay sockets.
[WARN] Failed to initialize netplay.

The sockets thing makes me think that perhaps there is something funny about how the flatpak version is built, but checking here, I do see the proper --share=network arg and creating and accessing sockets would also matter when hosting, which I can do.

So why can’t I join any games, only host?