This does not have to be the case. Remove that knowledge from the application/kernel and then you do not need anymore PORT/PASV.if you remove the knowledge, FTP client on IPv6-only machine cannot transfer files from/to IPv4 FTP server (since IPv4 FTP server does not understand EPSV/EPRT, with protocol #2).
Not if your FTP client is IPv6-only!!! - Alain.