Interesting stuff!
If have done the following but I am not sure if the outcome is the same as you have mentioned above:
to /etc/sysctl.conf I added
net.ipv6.conf.default.stable_secret=71c3:0126:4b41:96a4:a98f:5323:5341:2be9
rebooted and the check shows:
sysctl -a | grep stable_secret
sysctl: error reading key 'net.ipv6.conf.all.stable_secret': I/O error
net.ipv6.conf.br-lan.stable_secret = 71c3:0126:4b41:96a4:a98f:5323:5341:2be9
net.ipv6.conf.default.stable_secret = 71c3:0126:4b41:96a4:a98f:5323:5341:2be9
sysctl: error reading key 'net.ipv6.conf.erspan0.stable_secret': I/O error
sysctl: error reading key 'net.ipv6.conf.eth0.stable_secret': I/O error
net.ipv6.conf.eth0.2.stable_secret = 71c3:0126:4b41:96a4:a98f:5323:5341:2be9
sysctl: error reading key 'net.ipv6.conf.eth1.stable_secret': I/O error
net.ipv6.conf.eth1.1.stable_secret = 71c3:0126:4b41:96a4:a98f:5323:5341:2be9
sysctl: error reading key 'net.ipv6.conf.gretap0.stable_secret': I/O error
sysctl: error reading key 'net.ipv6.conf.ifb0.stable_secret': I/O error
sysctl: error reading key 'net.ipv6.conf.ifb1.stable_secret': I/O error
sysctl: error reading key 'net.ipv6.conf.lo.stable_secret': I/O error
sysctl: error reading key 'net.ipv6.conf.sit0.stable_secret': I/O error
net.ipv6.conf.wlan0.stable_secret = 71c3:0126:4b41:96a4:a98f:5323:5341:2be9
net.ipv6.conf.wlan1.stable_secret = 71c3:0126:4b41:96a4:a98f:5323:5341:2be9
The /etc/sysctl.conf still shows one line only. I am running the Netgear R7800