Hi hygap, thanks for sharing your experience
I' have the same problem and tried your solution, but unfortunately Its doesn't working for me. I'm on Prestashop 1.6.1
I have followed the steps:
1 - Create the product via web service. OK.
2 - Prepare the update process, passing my new value to the position_in_category field, for example 14
3 - Unset the associations->categories node.
4 - Call to the update process
And I always get an "1" in the response for the position_in_category field, instead of my value previously passed.
I also tried to remove the associations node completely, but with the same result.
I'm stuck, and really need to order my products via web service. Can anyone help me?
Thank you