Hi,

We hit this with the slight difference of redundant controllers never getting
operative. Juju version is 2.2.6 as stated in the output:

$ juju controllers
Use --refresh flag with this command to see the latest information.

Controller  Model    User   Access     Cloud/Region  Models  Machines   HA  Version
my_controller*      default  admin  superuser  my_region              2       142  1/3  2.2.6

$ juju status -m controller
Model       Controller  Cloud/Region  Version  SLA
controller  my_controller       my_region         2.2.6    unsupported

App  Version  Status  Scale  Charm  Store  Rev  OS  Notes

Unit  Workload  Agent  Machine  Public address  Ports  Message

Machine  State    DNS             Inst id  Series  AZ   Message
0        started  X.X.X.18    8dghdf   xenial  FC1  Deployed
42       started  X.X.X.62   tyqpxr   xenial  FC2  Deployed
43       started  X.X.X.29  d4dx38   xenial  FC3  Deployed

$ juju ssh -m controller 42

Lessing the machine log I see:

...
2017-11-24 12:09:28 DEBUG juju.worker.dependency engine.go:504 "state" manifold worker stopped: cannot connect to mongodb: no reachable servers
2017-11-24 12:09:28 ERROR juju.worker.dependency engine.go:551 "state" manifold worker returned unexpected error: cannot connect to mongodb: no reachable servers
2017-11-24 12:09:28 DEBUG juju.worker.dependency engine.go:553 stack trace:
no reachable servers
github.com/juju/juju/state/open.go:527: cannot connect to mongodb
github.com/juju/juju/state/open.go:103: 
github.com/juju/juju/worker/state/manifold.go:64: 
2017-11-24 12:09:31 DEBUG juju.cmd.jujud machine.go:1632 mongo is already initialized
2017-11-24 12:09:31 INFO juju.state open.go:129 opening state, mongo addresses: ["localhost:37017"]; entity machine-42
2017-11-24 12:09:31 DEBUG juju.state open.go:130 dialing mongo
2017-11-24 12:09:31 DEBUG juju.mongo open.go:173 dialled mongodb server at "127.0.0.1:37017"
2017-11-24 12:09:32 DEBUG juju.mongo open.go:173 dialled mongodb server at "127.0.0.1:37017"
2017-11-24 12:09:33 DEBUG juju.mongo open.go:173 dialled mongodb server at "127.0.0.1:37017"
2017-11-24 12:09:34 DEBUG juju.mongo open.go:173 dialled mongodb server at "127.0.0.1:37017"
2017-11-24 12:09:35 DEBUG juju.mongo open.go:173 dialled mongodb server at "127.0.0.1:37017"
2017-11-24 12:09:36 DEBUG juju.mongo open.go:173 dialled mongodb server at "127.0.0.1:37017"
...

$ sudo lsof -i | grep LISTEN
sshd     3657   root    3u  IPv4   21999      0t0  TCP *:ssh (LISTEN)
mongod  15856   root    6u  IPv4   22147      0t0  TCP *:37017 (LISTEN)

On primary node mongo I see the following:

juju:PRIMARY> rs.status()
{
        "set" : "juju",
        "date" : ISODate("2017-11-24T12:12:22.569Z"),
        "myState" : 1,
        "term" : NumberLong(42),
        "heartbeatIntervalMillis" : NumberLong(2000),
        "members" : [
                {
                        "_id" : 1,
                        "name" : "127.0.0.1:37017",
                        "health" : 1,
                        "state" : 1,
                        "stateStr" : "PRIMARY",
                        "uptime" : 2767413,
                        "optime" : {
                                "ts" : Timestamp(1511525542, 279),
                                "t" : NumberLong(42)
                        },
                        "optimeDate" : ISODate("2017-11-24T12:12:22Z"),
                        "electionTime" : Timestamp(1508758130, 1),
                        "electionDate" : ISODate("2017-10-23T11:28:50Z"),
                        "configVersion" : 94205,
                        "self" : true
                }
        ],
        "ok" : 1
}

juju:PRIMARY> rs.config()
{
        "_id" : "juju",
        "version" : 94205,
        "protocolVersion" : NumberLong(1),
        "members" : [
                {
                        "_id" : 1,
                        "host" : "127.0.0.1:37017",
                        "arbiterOnly" : false,
                        "buildIndexes" : true,
                        "hidden" : false,
                        "priority" : 1,
                        "tags" : {
                                "juju-machine-id" : "0"
                        },
                        "slaveDelay" : NumberLong(0),
                        "votes" : 1
                }
        ],
        "settings" : {
                "chainingAllowed" : true,
                "heartbeatIntervalMillis" : 2000,
                "heartbeatTimeoutSecs" : 10,
                "electionTimeoutMillis" : 10000,
                "getLastErrorModes" : {

                },
                "getLastErrorDefaults" : {
                        "w" : 1,
                        "wtimeout" : 0
                },
                "replicaSetId" : ObjectId("58f2d2d4824342d527fee1b2")
        }
}

juju:PRIMARY> db.machines.find({ _id: "27633ebe-aa9d-47f1-8465-f03fcd64a673:42" })
... "novote" : false, "hasvote" : false...
juju:PRIMARY> db.machines.find({ _id: "27633ebe-aa9d-47f1-8465-f03fcd64a673:43" })
... "novote" : false, "hasvote" : false...

So no extra nodes added to the replica set, no vote set for the new controllers.

José.