With my recent 17.01 build from two days ago, nlbwmon does report ipv6 stats. On the other hand, I have also noticed the absence of ipv6 data at least once after flashing a master build.
I guess that there may be some kind of startup race condition, or something, which may make nlbwmon to fail getting the ipv6 stats sometimes. Maybe something that @jow might look into. Delayed startup for nlbwmon?