Using pre_get_posts for meta value of LIKE comparison on ACF repeater sub field

You need to add Wildcards to your meta_value.

Change

$query->set('meta_query', array(
    array(
      'meta_key' => 'test_repeater_%_test_sub_field',
      'meta_value' => $search,
      'compare' => 'LIKE',
    ),
  ));

To

$query->set('meta_query', array(
    array(
      'meta_key' => 'test_repeater_%_test_sub_field',
      'meta_value' => '%'.$search.'%',
      'compare' => 'LIKE',
    ),
  ));