I don't see any proof that your client is somehow not accepting IPv6 connections/preferring IPv4 connections or malfunctioning in any way. There aren't that many users with IPv6 on their connections (and I've definitely never had a node connected via IPv6, either on the different VPS services I used or in my home connection). The probability of having IPv4 nodes connected is pretty higher.
Here are Google's IPv6 statistics. They're worth what they're worth... but it's a starting point to measure adoption.
Yes. There is nothing preventing my nodes to connect to any available IPv6 peers as I can manually initiate the connection to those peers. And there is nothing preventing any IPv6 peers from connecting to my nodes. However, I have an impression that something in Bitcoin node prefers IPv4 peers than IPv6 peers, hence my questions about this.
I think Google is not a good reference in term of IPv6 usage on Bitcoin node. According to
https://bitnodes.21.co/nodes/, there are 5705 nodes at the time I am writing this. And out that there are 4875 IPv4 nodes and 830 IPv6 nodes. So I was wondering that only maximum 3 of those 830 nodes, connect to each of my 2 nodes. I guess I just have to force my nodes to connect to those IPv6 nodes as many as possible.