What functions do you have disabled in the php.ini file that php-fpm is using?
Any functions included there cannot be enabled any where else.
So if you have proc_open disabled in your php.ini file, then you won't be able to enable it for any users in their php-fpm pool file.
If you want proc_open enabled for a particular user, then you have to remove proc_open from the php.ini file, then add it to all of the disabled_functions php_admin_value directive for all of the php-fpm pools. Then remove it from the disabled_functions php_admin_value for the pool that you want it enabled on.
Any functions included there cannot be enabled any where else.
So if you have proc_open disabled in your php.ini file, then you won't be able to enable it for any users in their php-fpm pool file.
If you want proc_open enabled for a particular user, then you have to remove proc_open from the php.ini file, then add it to all of the disabled_functions php_admin_value directive for all of the php-fpm pools. Then remove it from the disabled_functions php_admin_value for the pool that you want it enabled on.