Could you confirm if you are using the
CustomerService.getCustomers()? If so, then in order to retrieve all the client customers (with their clientCustomerIDs) associated to your OAuth2 credential, then do not set a specific clientCustomerId in your request header (or in your property file). If you'll manually set the clientCustomerId in your property file, then you run this method, only the details of the specific client customer will be returned.
Kindly do the suggestion first and let me know if this works.