When a PPPoE interface has dhcpv6-options (prefix delegation in my example) configured
but no explicit ipv6 node (such as ipv6 address autoconf or
ipv6 adjust-mss), the Jinja template writes noipv6 into the PPP peer
configuration. This prevents IPv6CP negotiation, causing the ISP's BNG to
silently ignore all DHCPv6 traffic. The dhcp6c process runs and sends
Solicit messages, but never receives a response.

There are no errors logged anywhere. The failure is completely silent as it also requires are restart (disconnect/connect) of the pppoe interface after adding the node, which led me into a few hours of debugging :/