update_post_meta not saving

Question

I'm a novice so please bear with me. Thank you.

• I created 2 custom post types. brands and models.
• I created relationship custom fields using acf under brands
• The relationship field pulls the posts from models
• Inside the models I created custom fields for vote counter. I used the custom field for the voting so we could see the vote count on each model in the backend as well as the frontend.

So this is the sequence

• brand post > Pulls model post (using relationship field) > model post pulls vote field

Now, this is where I'm having trouble.

I created a single post template for brand. The template displays all related data for a single brand and displayed the models related to the brand using foreach, this includes the vote field.

I created a script that will execute the ajax function on click to update the vote field. I used get_post_meta to get the car model vote field and update_post_meta to update vote field.

On click, the vote field number on the frontend went up but it wasn't saved. Everything is reset to 0 when the browser is refreshed. It's not saving on the backend either.

If you could enlighten me, I would really appreciate it. Thank you.

$data = [];
$id = (int)$_POST['post_id'];
$vote = get_post_meta($id, 'vote', true);

if( !$vote ) $vote = 0;
    
$vote = (int)$vote;
$vote++;
update_post_meta( $id, 'vote', $vote );
    
$data['vote'] = $vote;
        
die( json_encode($data) );