There has been alot of threads about this spanning months. Guild Hosts and Vice Hosts spend alot of time planning, then when GW starts the planning does not matter. It mostly seems to affect the AFK members. People who physically click the "fight" button at 8 PM generally go where they are assigned, though not always. As far as everyone else, it's a dice roll on where they end up.

One solution is to find out who in the guild is planning to be there at 8 PM, and base planning strategies on where to best place the actives, and just hope the AFK spread doesn't screw you over. Other than that, if you submit a trouble ticket all you will get as a response is "we are aware of this bug and are working to fix it. We apologize for any inconvenience."

However, as this has been a problem for a long time, just expect that it will not be fixed.