I have been experiencing the same issue since Alpha2 and I believe that the cause of the issue is an error in the auto-generated /tmp/filter_rules_ipv6 file on Ln 8 or Ln 21.
Code:
Ln 8->:PControls - [0:0]
Ln 21->-A FORWARD -i br1 -j WGNPControls
With guest network enabled, the rules fail to load due to an error at Ln 21 because the WGNPControls chain has not been defined. Because the rules fail to load, rules from /tmp/filter_ipv6.default remain in place which effectively disables IPV6 which I assume is the default configuration. If I update Ln 8 to :WGNPcontrols then load the rules using ip6tables-restore, the rules load just fine and IPV6 becomes fully functional. I've attached two versions of the filter_rules_ipv6 file, one with guest network on and one with guest network off. The only real difference between the two is that the -A FORWARD -i br1 -j WGNPControls line doesn't exist with guest network turned off which makes sense because the br1 interface doesn't exist.