Querying posts by ACF User field type

acf-user-field-type.php

<?php
$search_value = 'xyz'; // whatever
$field_value = sprintf( '^%1$s$|s:%2$u:"%1$s";', $search_value, strlen( $search_value ) );

$args = array(
  'post_type' => 'post',
  'meta_query' => array(
    array(
      'key' => 'user_field',
      'value' => $field_value,
      'compare' => 'REGEXP'
    )
  )
);