Marvel doesn't use the order of artboards inside Sketch, artboards are uploaded alphabetically:
The first time you upload your artboards:
Artboards will be added in alphabetical order. If you name your artboards a1, a2, b1, etc. the artboards will keep this order.
Adding more artboards later:
Artboards will be added in alphabetical order after your existing artboards. If you then add a3, b2, etc. the new order will be a1, a2, b1, a3, b2. This is to avoid overwriting any reordering you did within Marvel.