Something seems to trigger the next elif branch in interface.py L1284.
Your bug report suggest a malformed string in your set command.
Could you test with a quoted param?

My original pull request was using the 'dhcpv6' keyword under system interface pppoe pppoeN ipv6 address namespace.
@c-po enhanced that by matching global interface configuration by assigning the keyword to system interface pppoe pppoeN address to be more inline with general VyOS configuration.

pull request: https://github.com/vyos/vyos-1x/pull/4693

Partial solution for handling prefix delegation (ensuring graceful renew/rebind upon prefix lease timeout): https://vyos.dev/T7795#234773

Is there any dhcp6c repo (used as a dhcp6c base for vyos) where the issue can be forwarded?

dhcp6c seems very old: 20080615-24 https://manpages.debian.org/testing/wide-dhcpv6-client/dhcp6c.8.en.html

With nightly 2025.09.01-0023-rolling it is working (maybe the last kernel bump fixed it ?) :

Upon further investigation, dhcp6c doesn't even reset timers according to pltime !

Changing the base template controlling dhcp6c config generation seems to do the trick.
Folowing basic pppoe example provided on Vyos main website :

// config
interfaces {
    ...
    pppoe pppoe0 {
        ...
        dhcpv6-options {
            pd 0 {
                interface eth0 {
                    address "1"
                    sla-id "0"
                }
                length "56"
            }
            rapid-commit
        }
        ipv6 {
            address {
                autoconf
            }
        }
    }
}