WordPress | Change user Display Name to be their username after login

display-name-as-username.php

<?php 

// Replaces default display name to login name (username). As soon as the user logs in, the setting gets changed.

add_action( 'wp_login', 'fs_format_user_display_name_on_login' );

function fs_format_user_display_name_on_login( $username ) {
	$user = get_user_by( 'login', $username );

	$userdata = array(
		'ID' => $user->ID,
		'display_name' => $user->user_login,
	);

	wp_update_user( $userdata );

}