So I have been sitting on this data for a week - since the FOB NZGT ended.
I had published it to a TOs' group to see if they could explain what I was seeing but got the stock "not enough terrain/not the right sort of terrain" answer.
However this morning Goonhammer published an article which appears to confirm what I was seeing.
Put simply, if you go first you have a substantially increased chance of winning.
Goonhammer have indicated that the skew towards Go First Win is 58%. FOB NZGT had a more pronounced skew but if the average is 58%, within the bounds of uncertainty.
In their analysis, Goonhammer singled out Vital Intelligence and Surround & Destroy as particularly problematic. They also highlight Battlelines as the most balanced of the missions.
I'd certainly be using Battlelines as my Championship mission in future - last weekend it was fortuitous that that was the case.
Personally, I think this skew is worrisome. That there is such an advantage from a random die roll is something GW should be worried about.
Hopefully, this will be addressed in coming months,