Disable Source Filtering/Routing for IPv6 with PPPOE

In another discussion we have identified a problem where IPv6 source routing is causing an issue with Wireguard VPN . Does anyone know how to disable IPv6 Source Filtering / Routing on a PPPOE WAN setup with "Auto" IPV6 configuration (/56 dynamic prefix delegation)? Just unchecking the "IPv6 Source Routing" option in LuCI doesn't seem to set this option on the spawned wan_6 Virtual dynamic interface (DHCPv6 client).