When I setup replication between a master and slave where they are both using auth and run
db._adminCommand(

);
in the output i get the following

"repl" : {
"ismaster" : 0,
"msg" : "not paired",
"sources" : [
{
"host" : "10.10.0.1",
"syncedTo" :

,
"masterFirst" : "Thu Jan 01 1970 12:16:47 GMT+0000 (UTC)",
"masterLast" : "Thu Jan 01 1970 12:16:47 GMT+0000 (UTC)",
"lagSeconds" : 18446742793942500
}
]
},

To create the issue i start by setting up a master with the following config

1. cat /etc/mongod.conf
   dbpath = /db/mongodb
   logpath = /db/logs/mongodb.log
   fork = true
   bind_ip = 0.0.0.0
   master = true
   directoryperdb = true
   syncdelay = 5
   oplogSize = 51200
   auth = true

then log into the admin database locally create an admin user, then auth and switch to the local database and create the repl user.

Then start a slave with the following config

1. cat /etc/mongod.conf
   dbpath = /db/mongodb
   logpath = /db/logs/mongodb.log
   fork = true
   bind_ip = 0.0.0.0
   slave = true
   source = 10.10.0.1
   directoryperdb = true
   syncdelay = 5
   auth = true

Login and create the local repl user.

Import some data on the master

then run
> db._adminCommand(

);
and get the output above

Logs are attached