Access multidimensional array

$gallery = get_post_meta( $post->ID, 'ds_product_gallery', true ); 

Here is the markup

<div class="field_left">
  <div class="form_field">
     <label>Image URL</label>
     <input type="text" 
            name="meta[ds_product_gallery][picture][<?php echo $i; ?>]"
            value="<?php esc_html_e( $gallery['picture'][$i] ); ?>"
            type="url" pattern="https://.*" 
            size="30" 
            minlength="8" 
            placeholder="No picture chosen yet!" readonly
            />
  </div>
  <div class="form_field">
     <label>Description</label>
     <input type="text" 
            name="meta[ds_product_gallery][figcaption][<?php echo $i; ?>]"
            value="<?php esc_html_e( $gallery['figcaption'][$i] ); ?>"
            placeholder="Enter the caption here"
            />
  </div>

to add fields dynamically, i use this

 <div class="field_left">
      <div class="form_field">
          <label>Image URL</label>
          <input class="meta_image_url" 
                 name="meta[ds_product_gallery][picture][]" 
                 value="" 
                 type="url"  
                 size="30" 
                 minlength="8" 
                 placeholder="No picture chosen yet!" readonly />
      </div>
      <div class="form_field">
          <label>Description</label>
          <input class="meta_image_desc" 
                 value="" 
                 type="text" 
                 name="meta[ds_product_gallery][figcaption][]" 
                 placeholder="Enter the caption here"/>
      </div>
   </div> 

to send data to the database i use this

  if ( isset( $_POST['meta'] ) ) { 
      if ( $_POST['meta']['ds_product_gallery'] )
      {
         $gallery_data = array();
         for ($i = 0; $i < count( $_POST['meta']['ds_product_gallery']['picture'] ); $i++ )
                {
                    if ( '' != $_POST['meta']['ds_product_gallery']['picture'][ $i ] )
                    {
                        $gallery_data['picture'][]    = $_POST['meta']['ds_product_gallery']['picture'][ $i ];
                        $gallery_data['figcaption'][] = $_POST['meta']['ds_product_gallery']['figcaption'][ $i ];
                    }
                }

                if ( $gallery_data ) {
                    $_POST['meta']['ds_product_gallery'] = $gallery_data;
                }else{
                 unset($_POST['meta']['ds_product_gallery']);
                }
            
            }else{
                unset($_POST['meta']['ds_product_gallery']);
            }
            foreach( $_POST['meta'] as $key => $value ){
                update_post_meta( $post_id, $key, $value );
            }
        }
    

everything works great except from when i delete an element that was saved in the DB using javascript.

i am trying to figure out where my mistake since the weekend.