Ok, so it's probably using NetworkManager. I would try disabling it in /etc/NetworkManager/NetworkManager.conf by adding a block like:
[ipv6]
addr-gen-mode=stable-privacy
method=disabled
Then sudo systemctl restart NetworkManager.
Can't say for sure if this will work. I dislike using NetworkManager on my servers so I can't test if this works. But hopefully the before/after of ip addr is different.
Although it looks like your ip addr output posted an hour or so ago doesn't show any ipv6 addressing. Maybe the problem is solved now.