HelpFixDecoApp wrote @evilbunny Were you having the same issues before trying the beta firmware? Yes, and upgrading alone didn't solve the issue. Things seem fine now, except for the IPv6 firewall...

I've already installed the v1.2.0 firmware, as per my OP, when I was getting high packet loss over IPv6, and it didn't improve things. So I tried switching to stateless on the WAN and you have to...

The model on the bottom of the Deco is Deco BE65 Pro(US) Ver: 1.0 There is 1 very annoying thing whenever I make changes to the IPv6 internet connection, it deletes/removes all the firewall entries...

I've just setup my new deco mesh routers for IPv6 and noticed a couple of weird things, but mostly not fatal. Firstly when I set the WAN port to DHCPv6, it wouldn't show the WAN IP or DNS server IPs...