I'm having this exact problem too and have found this note written in the merchandisingService.wsdl

Important: This API call is no longer actively managed and cannot be relied upon to retrieve current product data, and in some cases, will not retrieve any data. As an alternative to this call, we recommend using the https://developer.ebay.com/api-docs/buy/marketing/resources/merchandised_product/methods/getMerchandisedProducts getMerchandisedProducts call of the Buy Marketing API