I can't speak to everything. I just know what I see in my death recap, and it's always molten lol
I'm not surprised. Molten is generally easier in PvP because you can proc the bonus without hitting anyone. The players I see in PvP using the Flame morph are built for using it.
Most everyone will either dodge / block any whip skill that's cast; it's not as though one morph is more prone to being blocked than the other, only a noob would stand there and take it.
But most DK's know that experienced players who even work together know better than to bunch-up; it's most often a ball group strat whereby one or more of their DK's are kitted for Lash within their group.