Customer Creation with API but Customer ID is Null at sometimes.

[Mike1232]

Customer API : https://shopify.dev/api/admin/rest/reference/customers/customer#create-2021-07.

Sometimes, Customer ID is null. Our shop : https://tupperware-kr.myshopify.com/

Customer ID is null but eMail would be registered at our shop.

Would you please check what's wrong with it?

customer id   eMail

NULL             ysmk7557@naver.com

NULL             dbswn1586@naver.com

NULL             emily65@hanmail.net

NULL             Cssgaetti71@hanmail.net

NULL             smnhk@hanmail.ner

NULL mimiokja@nate.com _