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.
Relevant PR:
https://github.com/vyos/vyos-1x/pull/1498