That style has come together really nicely
The problem you're seeing here with the material IDs is probably because the style has the option to use the Generator's Reverse
feature. This changes the direction of the spline but it also reverses the allocation of the material IDs, which can be confusing because they don't match what you've allocated to the spline itself.
In this case I'd recommend disabling reverse in the style (or as you've exported the value you can untick inside/outside
in your style's parameters) and changing the direction manually by right clicking on the spline object and selecting Reverse spline. You might also be interested in a little script I wrote a while ago that also allows you to reverse a spline while maintaining the material ID allocation. You can get it here.
I hope that helps, please let me know if you need anything further.