Adding custom item data to order when order is placed for the bookable product.

functions.php

<?php
/**
 * Adding custom data to item when booking order is placed.
 * 
 * @param int   $item_id Item ID.
 * @param int   $product_id Product ID.
 * @param array $booking_data Booking data.
 */
function bkap_update_item_meta( $item_id, $product_id, $booking_data ) {

	$order_id = wc_get_order_id_by_order_item_id( $item_id );

	// Start Date.
	$date_booking = bkap_date_as_format( $booking_data['hidden_date'], 'm-d-Y' );
	wc_add_order_item_meta( $item_id, 'Custom_Field_1', sanitize_text_field( $date_booking, true ) );
	update_post_meta( $order_id, 'Custom_Field_1', sanitize_text_field( sanitize_text_field( $date_booking, true ) ) );

	// End Date.
	$date_booking = bkap_date_as_format( $booking_data['hidden_date_checkout'], 'm-d-Y' );
	wc_add_order_item_meta( $item_id, 'Custom_Field_2', sanitize_text_field( $date_booking, true ) );
	update_post_meta( $order_id, 'Custom_Field_2', sanitize_text_field( sanitize_text_field( $date_booking, true ) ) );
}
add_action( 'bkap_update_item_meta', 'bkap_update_item_meta', 10, 3 );