Generating new customer on every new order

Questions
1

I have an issue in checkout API. I am sending customer id in generate checkout API and also getting in response bask. but after transaction payment and order showing with a new customer. it’s not linking to a real customer.

Request data:
{"idempotency_key":"51","redirect_url":"http:\/\/test.com\/","order":{"idempotency_key":"51","order":{"location_id":"LOCATION","customer_id":"XYZ","reference_id":"51","line_items":[{"name":"test2","quantity":"1","base_price_money":{"amount":1000,"currency":"USD"}}]}},"ask_for_shipping_address":true}

Response data:
{"checkout":{"id":"ABC-PQR","checkout_page_url":"https://connect.squareupsandbox.com/v2/checkout?c=CBASPQR-nHmIQ\u0026l=ABC","ask_for_shipping_address":true,"redirect_url":"http://test.com/","order":{"id":"ORDER ID","location_id":"LOCATION","reference_id":"51","source":{"name":"Sandbox for ABC"},"customer_id":"XYZ","line_items":[{"uid":"cLd7O","name":"test2","quantity":"1","base_price_money":{"amount":1000,"currency":"USD"},"variation_total_price_money":{"amount":1000,"currency":"USD"},"gross_sales_money":{"amount":1000,"currency":"USD"},"total_tax_money":{"amount":0,"currency":"USD"},"total_discount_money":{"amount":0,"currency":"USD"},"total_money":{"amount":1000,"currency":"USD"}}],"net_amounts":{"total_money":{"amount":1000,"currency":"USD"},"tax_money":{"amount":0,"currency":"USD"},"discount_money":{"amount":0,"currency":"USD"},"tip_money":{"amount":0,"currency":"USD"},"service_charge_money":{"amount":0,"currency":"USD"}},"created_at":"2020-05-04T07:28:46.586Z","updated_at":"2020-05-04T07:28:46.586Z","state":"OPEN","version":1,"total_money":{"amount":1000,"currency":"USD"},"total_tax_money":{"amount":0,"currency":"USD"},"total_discount_money":{"amount":0,"currency":"USD"},"total_tip_money":{"amount":0,"currency":"USD"},"total_service_charge_money":{"amount":0,"currency":"USD"}},"created_at":"2020-05-04T07:28:46Z"}}

2

With the Checkout API this is expected. The checkout will always create a customer itself outside of the context of the order passing an existing customer_id.