Meta Box clears saved field content

Question

I'm new in WP developing. I added metabox field to page, all works fine, but last one, which is similar with others, when I press save page button it clears all fields, even fields which were saved before. What I'm doing wrong? Or is it limitation in using metabox?

here is my code

add_action('add_meta_boxes', 'add_denet_faq_meta');

function add_denet_faq_meta(){
global $post;

if(!empty($post))
{
    $pageTemplate = get_post_meta($post->ID, '_wp_page_template', true);
    if($pageTemplate == 'faq.php' )
    {
        remove_post_type_support( 'page', 'editor' );
        add_meta_box(
            'economic_meta',
            'Economic and Tokenomic',
            'faq_economic',
            'page',
            'normal',
            'high');
    }
  }
}

function faq_economic(){
global $post;

wp_nonce_field( basename( __FILE__ ), 'page_meta_box_nonce' );
$faq_3_title = get_post_meta($post->ID, "_denet_faq_3", true);

$no_faq_3 = get_post_meta($post->ID, "_denet_no_faq_3", true);

$faq_3_question = get_post_meta($post->ID, "_faq_3_question", true);
$faq_3_question = ($faq_3_question != '') ? json_decode($faq_3_question) : array();

$faq_3_answer = get_post_meta($post->ID, "_faq_3_answer", true);
$faq_3_answer = ($faq_3_answer != '') ? json_decode($faq_3_answer) : array();
$counterX3 = 0;

echo '<input type="hidden" class="slide-fields x" id="val3" name="no_faq_3" type="number" value="'. $no_faq_3 .'" placeholder="0">';
echo '<h2 style="padding:0;"><input type="text" name="faq_3_title" style="width:100%;" placeholder="Tab Title" value="'. $faq_3_title . '"></input></h2>
<hr/>';

for ($ic = 0; $ic < $no_faq_3; $ic++ ) {
    if ($faq_3_question != " ") {
        echo '<div class="row">
<div class="col-md-4 col-sm-12"><input name="faq_3_question[]" style="width: 100%;" placeholder="Question" value="'. $faq_3_question[$counterX3] .'"></input></div>
<div class="col-md-7 col-sm-10" style="padding: 0;"><textarea name="faq_3_answer[]" style="width: 100%;" placeholder="Answer">'. $faq_3_answer[$counterX3++] .'</textarea></div>
<div class="col-md-1 col-sm-2"><span style="margin-top: 5px;" class="dashicons dashicons-no"></span></div>
</div>
<hr/>';
}}
echo '<input id="add3" name="add3" type="submit" class="btn sbmt tb" value="ADD MORE +"></input>';
}

add_action('save_post_page', 'denet_faq_meta_box_save');
function denet_faq_meta_box_save($post_id) {
if ( !isset( $_POST['page_meta_box_nonce'] ) || !wp_verify_nonce( $_POST['page_meta_box_nonce'], basename( __FILE__ ) ) ){
    return;
}
// return if autosave
if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ){
    return;
}
// check the user's permissions.
if ( ! current_user_can( 'edit_page', $post_id ) ){
    return;
}

if ( isset( $_REQUEST['no_faq_3'] ) ) {
    update_post_meta( $post_id, '_denet_no_faq_3', sanitize_text_field( $_POST['no_faq_3'] ) );
}

if ( isset( $_REQUEST['faq_3_question'] ) ) {
    $faq_3_question = (isset($_POST['faq_3_question']) ? $_POST['faq_3_question'] : '');
    $faq_3_question = strip_tags(json_encode($faq_3_question));
    update_post_meta($post_id, "_faq_3_question", $faq_3_question);

    $faq_3_answer = (isset($_POST['faq_3_answer']) ? $_POST['faq_3_answer'] : '');
    $faq_3_answer = strip_tags(json_encode($faq_3_answer));
    update_post_meta($post_id, "_faq_3_answer", $faq_3_answer); 
}

else {
    return $post_id;
}
}

Answer 1

I found it, it was actually clearing when inputting apostrophe (') which conflicts in database save, to solve I used htmlentities() adn js replace to replace and save apostrophe with apostrophe code.

<input onkeyup="valid(this)" type="text" name="faq_2_title" style="width:100%;" placeholder="Tab Title" value="'. htmlentities($faq_2_title) . '"></input></h2>

js:

function valid(f) {
  f.value = f.value.replace(/'/g, '&#39');
}