We had the same issue, using Plesk Onyx 17. Instead of messing up with rights etc., solution was to add nginx user into psacln group, in which all the other domain owners (users) were:
usermod -aG psacln nginx
Now nginx has rights to access .htaccess or any other file necessary to properly show the content.
On the other hand, also make sure that Apache is in psaserv group, to serve static content:
usermod -aG psaserv apache
And don't forget to restart both Apache and Nginx in Plesk after! (and reload pages with Ctrl-F5)