Ok, I got confused.
The issue is not in the category but products.
Please try and re-order the products and you'll find that i won't work.
list all poducts then filter them from the category drop down box so you can reorder them. You'll see that all products have a position 0 or nothing at all.
Another annoying issue is that when you filter the products the selection is not retained but this is another story and I believe someone else has already reported that

Thanks you for checking