I tried the proftpd-1.3.0a-1.fc6(kernel version is 2.6.19).
The ftp stop problem does not happen.
Therefore, this problem is reproduced when
client's kernel-version is 2.6.20-rc1 or later
and server is vsftpd.
Server's kernel-version is not related with this problem.
The ftp-stop-problem happens on client's PC.