[syslog-ng] Host sending "localhost" as hostname

Hello, we have a setup with Syslog-Ng 3.2 on both a debian server and Ubuntu clients. For several reasons, we set keep_hostnames(yes) on the log server, which works quite well. Except for some clients (3 or 4) which insist on sending "localhost" as their hostname. They do resolve perfectly via DNS in both directions, "hostname" on these clients outputs the correct hostname. I have no idea where this comes from. Configuration is identical on all machines, so I don't understand why it works on most but not all of them. Any Ideas? Thanks, Andreas