using latest rclone and ubuntu 18.04
created ftp config called config_ftp
set up env vars as below
export ftp_proxy=http://192.168.1.143:3128
export FTP_PROXY=http://192.168.1.143:3128
export http_proxy=http://192.168.1.143:3128
export HTTP_PROXY=http://192.168.1.143:3128
rclone ls config_ftp: -vv
rclone tries to connect directly to ftp server bypassing the proxy