IPv6 pubsub issue

During testing of the pubsub feature for IPv6 networks only, I detected a difference between
IPv4 and IPv6. (go-ipfs version: 0.4.19-dev-ca77ecc76)

Scenario 1 (IPv4 ):
Two nodes behind a router with non public IP addresses.
The config is --profile=lowpower (for both nodes)
and
"Addresses": {
"Swarm": [
"/ip4/0.0.0.0/tcp/4001"
],
...
}
The id() of the shows IPv4 addresses (though of course not public)
=> Using the pubsub feature works fine, both nodes detect each other
without knowing each others peer ID. [success]

Scenario 2 (IPv6 ):
Two nodes behind a router with non public IPv6 addresses (something with fdxx...).
The config is --profile=lowpower (for both nodes)
and
"Addresses": {
"Swarm": [
"/ip6/::/tcp/4001"
],
...
}
The id() of the shows IPv6 addresses (though of course not public, "fd....")
=> Using the pubsub feature is not working, both nodes do not detect each other. [failure]
=> but when at least one knows the other PID and a manual connection is made, then it is working

So here is the question? Is this a bug, a limitation. Has it something to do with the MDNS discovery mechanism?