There are intentional restrictions on forming a group in the Maw. Normally, grouping up will pull everyone into the group leader’s shard (assuming everyone’s Warmode is compatible). In the Maw, the phasing shift does not happen. To guarantee identical sharding, members of the group have to travel someplace else (say, Oribos) and back into the Maw as group members. When re-entering the Maw in the group, you will be sharded appropriately.
Let me emphasize: intentional. The development team was very clear that it was working exactly the way it’s supposed to. They never really clarified why they wanted it working that way, probably because they are literally under zero obligation to get the player’s approval. You don’t have to understand why and you don’t have to like it. You either put up with it or you don’t play that part of the game.