Cannot co-locate routers

The `base-port` configuration option appears to allow for the co-location of mysql-router applications. However I am having trouble doing so. I tried with the keystone and placement charms and the following messaging is surfaced:

Unit Workload Agent Machine Public address Ports Message
keystone/0* active idle 0 10.5.0.24 5000/tcp Unit is ready
  keystone-mysql-router/0* active idle 10.5.0.24 Unit is ready
mysql-innodb-cluster/0* active idle 1 10.5.0.5 Unit is ready: Mode: R/W
mysql-innodb-cluster/1 active idle 2 10.5.0.13 Unit is ready: Mode: R/O
mysql-innodb-cluster/2 active idle 3 10.5.0.16 Unit is ready: Mode: R/O
placement/1* blocked idle 0 10.5.0.24 8778/tcp 'shared-db' incomplete, 'placement' missing
  placement-mysql-router/1* waiting idle 10.5.0.24 MySQL Router not yet bootstrapped

$ juju config keystone-mysql-router base-port
3306

$ juju config placement-mysql-router base-port
3312

See attachment 'colocating-mysql-routers.txt' for details.