Redis session caching with PHP 7 and ZF2: Session object destruction failed

We have been caching sessions with Redis in our Zend Framework 2 project (using zend-session 2.5.2) on PHP 5.6.

After upgrading to PHP 7.0.4, there was a problem with redis session caching, resulting in the following error when users logged in: PHP message: PHP Catchable fatal error: session_regenerate_id(): Failed to create(read) session ID: user. But a custom save handler fixed it:

namespace OurFramework\Session\SaveHandler;

class Cache extends \Zend\Session\SaveHandler\Cache
{    
    public function read($id)
    {
        return (string)parent::read($id);
    }
}

The session config is as follows:

'session' => [
    'cookie_lifetime' => '172800', 
    'cookie_secure'   => '1',
    'cookie_httponly' => '1',
    'name'            => 'some-session-name',
    'save_handler'    => 'redis',
    'save_path'       => "tcp://$host:$port?weight=1&timeout=3",
],

However, we also get warnings like session_regenerate_id(): Session object destruction failed. ID: user (path: tcp://host:port?weight=1&timeout=3) and session_destroy(): Session object destruction failed when users are logged out.

What may be causing these warnings on PHP 7.0.4 and how can they be fixed?