Neutron's L3 agents stuck in standby/standby HA state after creating a new router

Issue

1. When an HA router is created...

   openstack router create --ha --project project_X router_Y
   

2. ...Neutron's L3 agents assigned to the HA-router remain in standby HA state indefinitely:

   openstack network agent list --router router_Y --long
   +-------------------------+------------+----------+-------------------+-------+-------+------------------+----------+
   | ID                      | Agent Type | Host     | Availability Zone | Alive | State | Binary           | HA State |
   +-------------------------+------------+----------+-------------------+-------+-------+------------------+----------+
   | {uuid}                  | L3 agent   | net01    | nova              | :-)   | UP    | neutron-l3-agent | standby  |
   | {uuid}                  | L3 agent   | net02    | nova              | :-)   | UP    | neutron-l3-agent | standby  |
   +-------------------------+------------+----------+-------------------+-------+-------+------------------+----------+