You have not posted the code that is seeing the ‘extra’ character so this is speculation. The publisher you have posted sends 4 bytes of payload as strlen does not include the null termination of a string. Any code that retrieves the payload will get 4 bytes of binary payload that happen to look like a string but is not a string as it is not null terminated. If you attempt to retrieve the payload and print it, it is quite likely that you will see garbage characters at the end, or crash.