This is expected behavior. As per the section Step 4: Create REST API with AMQP Outbound support of Event Driven APIs with API Gateway and AMQP
“When the API is invoked from the REST client, the API Gateway will route/post the input message to a Message Queue/Topic and look for the response in the ReplyTo Queue/Topic.”
API Gateway lacks control or access to the ReplyTo queue, making it the responsibility of the Native Endpoint to populate the queue. If the message has been successfully posted, it will serve as a response to the client; otherwise, an error will be triggered. The inability of APIGW to read the message results in a timeout error.