php, upload multiple files from multiple input

I have a form with multiple rows (rows from SQL, iterated and generated the form). Each of those rows has a multiple file input field (the user can upload 0, 1 or multiple in each input). Files are being uploaded correctly, with correctly assigned new file names (file names are generated based on each row SQL ID). Then after user click the SUBMIT, the file names are added to a column in SQL table.

The form can generate 0, 1 ... 100 rows, so to prevent the php iterating through all those 100s rows, and executing an UPDATE or rows where it is not needed, I only execute the UPDATE on rows that the user has touched.

My problem is when I upload a single or multiple file on a singular file input field (a row), then everything works as expected. However, when I try to upload 1 or more files in separate file inputs fields, then something strange happens in SQL, the files upload to the server are all OK, but database goes weird.

1. The first input (row) will have correct number of files with correct names, as desired.
2. Every next input (row, in the database after the UPDATE) will have the all the file names from all previous inputs, plus the ones that were uploaded for the specific input... if that makes sense... Here illustration:

Input 1: input1file1.jpg, input1file2.jpg
Input 2: input1file1.jpg, input1file2.jpg, input2file1.jpg

Because each input field can take multiple files, so the input name is already an array name='attachment[]'.

To distinguish between those input fields, I add row ID to file name.

Here is my code, I cannot work out what is causing it?

Form:

<form enctype="multipart/form-data" method="post">
...
php iteration:
  <div>
    <input class="hasChanged" name="attachments'.$row['pk'].'[]" type="file" multiple>
    <input value="'.$row['existing_attachments'].'" id="updateAttachmentValue'.$row['pk'].'" name="existingAttachments'.$row['pk'].'">
  </div>
...
</form>

PHP:

//iterate
foreach($_POST['foo_field'] as $key => $n){
  if(in_array($upl[$key], $touched_rows)){
    $aID = $upl[$key];
    $prevAttachments = (empty($_POST['existingAttachments'.$aID]) || $_POST['existingAttachments'.$aID]=='|') ? NULL : $_POST['existingAttachments'.$aID];
    $attachments = NULL;
    if(count($_FILES['attachments'.$aID]['name']) > 0){
      if(!empty($_FILES['attachments'.$aID])){
        for($i=0; $i<count($_FILES['attachments'.$aID]['name']); $i++){//Loop through each file
          $tmpFilePath = $_FILES['attachments'.$aID]['tmp_name'][$i];//Get the temp file path
          if($tmpFilePath != ''){//Make sure we have a filepath
            $shortname = 'AP-'.$aID.'-'.$_FILES['attachments'.$aID]['name'][$i];//save the filename
            $filePath = $attachmentDirectory.'\AP-'.$aID.'-'.$_FILES['attachments'.$aID]['name'][$i];//save the url and the file
            if(move_uploaded_file($tmpFilePath, $filePath)){//Upload the file into the correct dir
              $files[] = $shortname;
            }
            $attachments = implode('|',$files);
          }
        }
      }
    }
    $prevAttachments = !empty($attachments) ? $attachments.'|'.$prevAttachments : $prevAttachments;
    try{
      $stmt = $conn->prepare("EXEC [name].[dbo].[table] :p1,:p2");
      $stmt->bindParam(':p1', $upl[$key], PDO::PARAM_INT);
      $stmt->bindParam(':p2', $prevAttachments, PDO::PARAM_STR);
      $stmt->execute();
    }catch(PDOException $e){ echo 'ERROR: ' . $e->getMessage();}
  }
}