'Prefix of tables' value/entry is not used for creating tables in Wordpress database. Random value is taken in wp-cofig.php as well as in tables of database:
This issue with 'Wordpress Toolkit' is already known and was registered with ID PPPM-2056.
Plesk generates random prefix only if 'Wordpress Toolkit' is enabled (it depends on license key or setings in panel.ini):
cat /usr/local/psa/admin/conf/panel.ini (Windows: %plesk_dir%\admin\conf\panel.ini) .. [wpToolkit] enabled=off ...
The possible workaround is to disable Wordpress Toolkit. In this case table name will be 'myprefix' + 'wp_' + 'tablename'. For example:
mysql> show tables myprefixwp_comments
This is because configuration script in the Wordpress package has the following line:
$db_modify_hash['@@DB_MAIN_PREFIX@@'].='wp_'; // Means that db prefix will be "myprefixwp_".
There should not be any technical problems to remove adding 'wp_' from Wordpress configuration script.