It seems to be because its developers have not yet provided a version of the new version of Openwrt.
Maybe you can try to configure vsftpd via SSH?
[OpenWrt Wiki] FTP servers
Manpage of VSFTPD.CONF (security.appspot.com)
Thank you for your feedback, as the 4.x version of the firmware on flint is still in beta, it may have a lot of issues.