Returns NULL after loading from the database, why?

Question

PHP

Good day. There is a page for deleting from the database, I display 100 records as soon as I scroll to the last records, it starts loading the next 100 so that it does not buggy, since there are 90k records. I use "checkbox" to delete, what is the actual problem, if we select the first 2 records, and press the delete button, delete, but if you select the first 2 records from the next 100 loaded, then it returns NULL when deleting, why? And how can you fix it?
first 100 entries

if (isset($_POST['delete_data'])) {
          $cheked = $_POST['check'];
          foreach ($cheked as $del_id) {
            $del = mysqli_query($connect, "DELETE FROM `test_excel` WHERE `id` = $del_id");
          }
        }
          $result = mysqli_query($connect, "SELECT * FROM `test_excel` LIMIT 100");
          while ($row = mysqli_fetch_array($result)) {
            echo "<tr><td><input type='checkbox' name='check[]' value='".$row['id']."'></td><td>".$row['id']."</td><td>".$row['name']."</td><td>".$row['nameScore']."</td><td>".$row['organization']."</td></tr>";
          }

ajax request:

$(document).ready(function(){
var inProgress = false;
var startFrom = 100;

  $(window).scroll(function() {
    if($(window).scrollTop() + $(window).height() >= $(document).height() - 100 && !inProgress) {
      $.ajax({
          url: 'delete_data_ajax.php',
          method: 'POST',
          data: {
            "startFrom" : startFrom
        },
          beforeSend: function() {
          inProgress = true;
          }}).done(function(data){
          data = jQuery.parseJSON(data);
          if (data.length > 0) {
          $.each(data, function(index, data){
          $(".table_delete").append("<tr><td><input type='checkbox' name='check[]' value='"+ data.id +"'></td><td>"+ data.id +"</td> <td>"+ data.name + "</td> <td>" + data.nameScore + "</td> <td>" + data.organization + "</td></tr>");
        });
            inProgress = false;
            startFrom += 100;
        }});
      }
  });
});

file "delete_data_ajax.php"

$startFrom = $_POST['startFrom'];
   $res = mysqli_query($connect, "SELECT * FROM `test_excel` LIMIT {$startFrom}, 100") or die('нет подключение к бд');

   $resSeacrh = array();
   while ($row = mysqli_fetch_assoc($res)) {
       $resSeacrh[] = $row;
   }
   echo json_encode($resSeacrh);

Answer 1

1. Use browser debugging, see what your AJAX sends. You can also just send data with the same Postman.
   
2. See what comes in PHP, again, you can output data for debugging.
   For example echo "SELECT * FROM `test_excel` LIMIT {$startFrom} , 100" ;