When the `default-route` parameter is set to `none` for a PPPoE interface, installation of a default route is correctly suppressed for IPv4. However, given that IPv6 obtains it's routes directly via the kernel via router advertisements, a default route is installed regardless of the aforementioned setting (if router advertisements on the PPPoE interface advertise a non-zero router-lifetime, the kernel adds a default route via the advertising router).
Setting `default-route` to `none` should also suppress installation of the IPv6 default route, irrespective of any received router advertisements on the PPPoE interface.