Yeah, basically the same as bug #619557, xl2tpd upstream still doesn't support IPv6, so IPv6 support was never added to NetworkManager-l2tp.
NetworkManager-l2tp >= 1.20.0 can use kl2tpd from the Katalix go-l2tp package for L2TP support: https://github.com/katalix/go-l2tp Katalix are the ones that wrote the L2TP Linux kernel modules (that xl2tpd and kl2tpd make use of). Unlike xl2tpd, kl2tpd supports IPv6. There is no Debian package that comes with kl2tpd yet, but the upstream kl2tpd author is planning on providing one. But having said that, network-manager-l2tp source code will need to be modified to support IPv6 with kl2tpd, pppd and the optional IPsec daemons. There is a chance the next version of network-manager-l2tp might support IPv6 if kl2tpd is used (any github pull requests for IPv6 support are welcome). You can use the kl2tpd from the command-line if you need IPv6 support now.
