Gluster geo-replication status shows fewer master volumes than expected

Issue

When displaying the status of a 2x2 distributed-replicate volume, which is geo-replicated, only 3 master volumes are shown (4 are expected):

# gluster volume geo-replication VOLUME status

MASTER NODE  MASTER VOL  MASTER BRICK  SLAVE USER  SLAVE                      SLAVE NODE STATUS  CRAWL STATUS    LAST_SYNCED                  
----------------------------------------------------------------------------------------------------------------------------------------------------------
node4        volname     /vol/brick    root        ssh://node5::volume_slave  N/A        Faulty  N/A             N/A                          
node1        volname     /vol/brick    root        ssh://node5::volume_slave  node5      Active  Changelog Crawl 2016-08-03 13:12:44          
node2        volname     /vol/brick    root        ssh://node6::volume_slave  node6      Passive N/A             N/A