Thank you for the explanation. That means the feed is providing the correct URL, i.e.
https://www.example.com/en/products/84/sample-product?combination=143
However, Google complains (at least, this is what the ad agency is saying) about the redirect that then happens to the URL with the # and is unable to index the page. I get a lot of errors saying that the page is not accessible to Googlebot.
If this behaviour is standard, I don't understand how Google can be having problems because of the redirect from the URL with the ? to the one with the #. This would mean that all thirtybees/Prestashop users are having problems.
Which would mean the ad agency is wrong in thinking that's the reason why Google can't index.