Hi, I found out that the endpoint that links products to deals was missing the product_id argument from the Pipedrive API docs. The endpoint itself is working as you would expect.
In technical terms it’s about post request to the endpoint /deals/{id}/products.
The Pipedrive dev team has been contacted and this should be fixed soon. Just wanted to inform others that have been scratching their heads.
You can read further explanations about the product-deal issue from my blog.