Cross-play is not working

[WFTR]

I'm unable to play multiplayer with a friend. I'm on Steam and he's on PlayStation. We added each other as friends via Epic Games.

In the game, I create a party and send him an invite through the Epic Games overlay. He receives the invite and accepts it. On my screen, it shows that the party is now 2/3. I launch Quick Race, but nothing happens—my friend remains marked as "not ready."

Meanwhile, on his screen, the party appears completely empty, and he cannot create his own party either. However, when I select Quick Race, all other game modes become locked for him. If he tries to start Quick Race on his end, the game starts for him, but only in spectator mode, while nothing happens on my end.