You may be seeing this because [SENDWITHUS TEST] is appended to all preview sends. This is also the result of sends using a Test API Key. To start using a Production API Key, simply modify the API key in your code - Production API Keys can be found in API Settings. Learn more about API Keys here.

Some email clients (like Gmail) will thread similar emails together, the most reliable way to make sure your subject hasn’t changed is by checking the email source.