Promoted redis master still thinks it's slave

Question

I'm using a redis setup with a master, two slaves and a single instance of redis sentinel ( i know this is not recomended, but we are just doing a proof of concept).

The master listens on 6379. When we failover the master, we query redis sentinel and now my slave is the master, so let's say that 6380 is now the master.

127.0.0.1:46379> sentinel masters
1)  1) "name"
    2) "redis6379-master"
    3) "ip"
    4) "127.0.0.1"
    5) "port"
    6) "6380"

But when I try to write to the instance on port 6380, it says it can't write to it because it's a slave. So when I query that instance, this is what I get (partially truncated for simplicity):

# Replication
role:slave
master_host:127.0.0.1
master_port:6380
master_link_status:down
master_last_io_seconds_ago:-1
master_sync_in_progress:1
slave_repl_offset:1
master_sync_left_bytes:-1
master_sync_last_io_seconds_ago:10
master_link_down_since_seconds:531
slave_priority:100
slave_read_only:1
connected_slaves:0
master_repl_offset:0
repl_backlog_active:0
repl_backlog_size:1048576
repl_backlog_first_byte_offset:0
repl_backlog_histlen:0

It seems like sentinel isn't being able to reconfigure the slave to act has a master? Any clues?

Thanks, Pedro