wpum_alert_admin_user_field_change.php

<?php

add_filter( 'account_page_form_fields', function ( $fields ) {
	if ( ! wp_verify_nonce( $_POST['account_update_nonce'], 'verify_account_form' ) ) {
		return $fields;
	}

	/**
	 * Changed this array to the field keys (wpum_ is the prefix)
	 */
	$field_keys_to_alert = array(
		'wpum_custom_field',
	);

	$changed_data = array();

	foreach ( $fields as $group_key => $group_fields ) {
		foreach ( $group_fields as $key => $field ) {
			if ( ! in_array( $key, $field_keys_to_alert ) ) {
				continue;
			}
			if ( isset( $_POST[ $key ] ) && $_POST[ $key ] !== $field['value'] ) {
				$changed_data[ $key ] = array( 'old' => $field['value'], 'new' => $_POST[ $key ] );
			}
		}
	}

	if ( empty( $changed_data ) ) {
		return $fields;
	}

	$to      = get_option( 'admin_email' );
	$subject = 'Alert: User account updated ';

	$user = get_user_by( 'id', get_current_user_id() );

	$message = esc_html__( 'The following user has updated their account' ) . "<br><br>";
	$message .= sprintf( esc_html__( 'Username: %s' ), $user->user_login ) . "<br>";
	$message .= sprintf( esc_html__( 'E-mail: %s' ), $user->user_email ) . "<br>";
	$message .= sprintf( esc_html__( 'First Name: %s' ), $user->first_name ) . "<br>";
	$message .= sprintf( esc_html__( 'Last Name: %s' ), $user->last_name ) . "<br>";
	$message .= sprintf( esc_html__( 'Website: %s' ), $user->user_url ) . "<br><br>";

	foreach ( $changed_data as $key => $values ) {
		$message .= sprintf( esc_html__( '%s:', 'wp-user-manager' ), $key ) . "<br>";
		$message .= sprintf( esc_html__( 'From: %s', 'wp-user-manager' ), $values['old'] ) . "<br>";
		$message .= sprintf( esc_html__( 'To: %s', 'wp-user-manager' ), $values['new'] ) . "<br><br>";
	}

	wp_mail( $to, $subject, $message, array( 'Content-Type: text/html; charset=UTF-8' ) );

	return $fields;
} );

I can get the current value of a user's ACF field using this $user->site_name (site_name being the name of the ACF field) but if I put 'site_name' in the $field_keys_to_alert array and then change the value using the account page I don't seem to receive an email alert at all. Not too sure what else to try for this