Time field value resetting to zero

I'm having some issues with a Meta Box field with the "time" type.
The field value is saving properly, and is showing correctly in the admin column.
But when I edit the custom post, the time is set to 00:00:00. So when I save again, this value is saved, thus resetting the time value.
What can be the problem here?

This is the meta field code:

array(
						'name'  => __( 'Playing time', 'KEY' ),
						'id'    => $prefix . 'rel_duration',
						'type'  => 'time',
            'timestamp' => false,
						'js_options' => array(
							'stepMinute'        => 1,
							'timeFormat'      => 'HH:mm:ss',
							'controlType'			=> 'select',
							'showButtonPanel' => false,
							'timeInput'				=> true,
							'timeOnlyTitle' => __('Playing time', 'KEY'),
						),
						'admin_columns' => true,
				),