Amazon SNS message saying error "Message body is invalid"

Yes. It is using Amazon SDK API to send message. As I said, some messgaes are being delived and many messages are giving this error. If message body will be improper than it might happen for all the messages.

Have you logged the message body for each message before you sent? e.g.,

const messageBody = readFromTemplate(entityId, templateId);
console.log(messageBody); // logging for debug

// Send message
const publishCommand = new PublishCommand({
  Message: messageBody,
  // ... rest of args
});
await snsClient.send(publishCommand);

You might notice incorrect message body for some of the message.