I had the same problem. In my case it was related to the theme I was using (United Themes Brooklyn in the latest version 3.3). I returned to a previous version of the Brooklyn theme and it worked again.
Ok, So the problem was associated with the United Themes Brooklyn v3.3 theme. I also recently installed. I returned to an older Brooklyn version and the products in the categories are displayed correctly again.
Ok, So the problem was associated with the United Themes Brooklyn v3.3 theme. I also recently installed. I returned to an older Brooklyn version and I can add the products correctly again.
