API Gateway - before data transformation, AWS could return complete payload from endpoint, however after transformation it becomes incomplete

Before data transformation, the endpoint response 539982 lines json which is complete However in our case, we need do data transformation via mapping template before it response to customer. Basically, in the mapping template, it is a loop to retrieve valid data. we found the return payload is incomplete and end with "," . Response code is 200. How can we handle it?