I might have found the culprit, it turns out to be a bug that will be fixed in future releases.
I compared the output of the HTTP Request from Postman to the one run from. The problem is the AcceptFormat property of the HTTP Request activity:
It overwrites the header specified here:
As such, when you compare the output of Studio POST request and Postman POST request, the accept header that reaches the server when sent via Studio is different.
Unfortunately, there is no way around with the current Web activity package version and you will need to wait for a patch.
It is possible to achieve it if you would code your own HTTP Request by using the Invoke Code activity.
Having found the reason it doesn’t work, I would suggest you to expire your API credentials you have posted on our Forum.