Same issue. I have 2 friends that i want to play with, one on Xbox and one on PS5, and I'm also on PS5. All 3 of us confirmed crossplay is enabled. When i'm party leader, i can have just the xbox guy in my party and we can do everything normally. When the xbox guy is party leader and i'm the only other one in the party, he is locked out of all game modes ("Unavailable with party"). Whenever the other PS5 guy is in the party, doesn't matter who is party leader, we get locked out of all game modes. All of us confirmed our respective online multiplayer subscriptions are active and we have no issues playing multiplayer bf6 when we are not in a party. Restarting the game, and/or console, has no effect.
Right now, for all 3 of us, this is by far the worst bug that is killing our enjoyment the most.