Can you share your workbook?
This is because Product Count is a discrete measure, Tableau does not allow grand totals on some of discrete measures specifically if the field is not coming from a primary data source.
also you can try a few other things such as creating 2 separate sheets one with the normal calculations and another one with the product counts and then align them in a dashboard or something like this if you prefer
Thank you for letting me know that Tableau does not allow grand totals on some of discrete measure..
Create a different layout and have achieve the expected result..