The RFC isn't clear whether the feature is enabled if the UTF-8 string is found in the FEAT response. All it says is that the server supports it. If you look at other commands which are announced in the FEAT reply you will notice that it doesn't mean that they are automatically enabled by default. Also why should the server (with sending the UTF8 string in the FEAT reply) assume that the FTP client supports/uses UTF8?
ProFTPD should, as any other FTP server which supports UTF-8, implement the OPTS command to explicitly enable/disable it.