Hi,
In /usr/share/vyos/templates/accel-ppp/config_modules_ipv6.j2 there is a conditional:
{% if ppp_options.ipv6 is defined and ppp_options.ipv6 != 'deny' %}
This can never be set as this option is not supported. Supposedly config_ipv6_pool.j2 uses
{% if client_ipv6_pool is defined and client_ipv6_pool is not none %}
for same purpose. So please either add option or edit the template.