I have the same issue. After the first night of playing every multiplayer game I play with my group of friends, pairs us off in the same teams. If there a 3 or 4 people in the group the same 2 players are paired off. 1 person is on Xbox with the other 2 on PC. The pairing is always the same with an Xbox player and PC player together, with the other 2 people on PC paired together or solo with a random fill in.
We've tried joining the group in different orders, differnt party hosts, even playing games without all members present... and it is always the same grouping once 3 or 4 of us are together.