Why a class property becomes empty?

Question

I have a class with a private property/variable named $postID that I can pass successfully (this was tested) to every function within this class except one named handleForm(). When it is passed to this function it becomes empty. Why this happens? I suspect that this is because of the template_redirect action hook, but I am not sure. The class schematically looks as below. To understand this class better you can look at the original code: https://wordpress.stackexchange.com/a/210662/25187.

UPDATE

For more understanding: I added to a front end posting form the editing functionality, but I want to solve some issues, although this form already works.

class Post_From_Front {
    ...
    private $postID;

    function __construct() {
        if( 'POST' == $_SERVER['REQUEST_METHOD'] && isset( $_POST['postid'] ) ) {
            $this->postID = $_POST['postid'];
        }
        ...
        // Listen for the form submit & process before headers output
        add_action( 'template_redirect',  array( $this, 'handleForm' ) );
    }

    function post_shortcode() {
        ...
    }

    function handleForm() {
        ...
    }

    function getForm() {

        ob_start();
        ?>
           // a form here
        <?php
        return ob_get_clean();
    }

}

new Post_From_Front;

The full code can be seen here: https://gist.github.com/iuriemalai/5bad5e05c92b1cec10db