What's the problem?

Have you reported it on the nmap-dev mailing list with packet dumps?

As per the documentation http://nmap.org/6/ :

We also felt that the old -sP ("ping scan") option was a bit misleading because current versions of Nmap can go much further (including -sC and --traceroute) even with port scans disabled. We will retain support for the previous option names for the foreseeable future.

That command works great for me using nmap 6. It finds all hosts on my network. And now with v6, I can do something similar to find IPv6 addresses on my network, with this command: sudo nmap -6 -sL --script targets-ipv6-\* --script-args=newtargets