When a server shows with a PID of 0, it means that the broker has been restarted since that server registered with it. Because servers register with the broker and not vice versa, this information is only available when a server first registers. If the broker is restarted, it does not contact the server and request the PID.
In order to have the PIDs correctly represented in the broker, you could use the brcontrol delete_dm and add_dm options to remove the server from the broker and add it back with the PID. For more information on the brcontrol options, please see brcontrol --help.