Login issue; Password gets a response code of "331 OK"

[BproB]

Scenario: Connecting to an ftp server using a Proxy
Issue: Login fails intermittently
Error: Password gets a response of 331 OK

Output from the terminal:

2018/12/12 11:23:03 < 220 TEST Server
2018/12/12 11:23:03 220 TEST Server
2018/12/12 11:23:03 > USER test
2018/12/12 11:23:03 < 331 OK
2018/12/12 11:23:03 > PASS test
2018/12/12 11:23:03 < 331 OK

This all happens in a test file where I am creating an FTP server, and trying to connect to it through proxy. We are testing for a couple of scenarios; upload, download, and file stats. After a couple of successful runs, Login spits out the above error; wherein it expects some input from the user. I tried overcoming this by changing Login function this way:

func (ftp *FTP) Login(username string, password string) (err error) {
	if _, err = ftp.cmd("331", "USER %s", username); err != nil {
		if strings.HasPrefix(err.Error(), "230") {
			// Ok, probably anonymous server
			// but login was fine, so return no error
			err = nil
		} else {
			return
		}
	}
	if _, err = ftp.cmd("230", "PASS %s", password); err != nil {
		if strings.HasPrefix(err.Error(), "331") {
			// Ok, probably anonymous server
			// but login was fine, so return no error
			err = nil
		} else {
			return
		}
	}

	return
}

This ignores "331 OK" and moves forward. But when the test starts for Upload; after the first instruction upload function sends, it gets a response for the password: "230 Password ok, continue"

2018/12/12 11:23:03 < 220 TEST Server
2018/12/12 11:23:03 220 TEST Server

2018/12/12 11:23:03 > USER test
2018/12/12 11:23:03 < 331 OK
2018/12/12 11:23:03 > PASS test
2018/12/12 11:23:03 < 331 OK

=== RUN   TestUpload
In TestUpload()
2018/12/12 11:23:03 > TYPE I
2018/12/12 11:23:03 < **230 Password ok, continue**
2018/12/12 11:23:03 > STAT sample_for_ftp.txt
2018/12/12 11:23:03 < 200 Type set to binary
x
Failures:

The hacky fix that I came up with to overcome this issue was to re-login to the ftp server during the setup but I need to understand why this is happening as it is weird, and defies the underlying ftp concepts.

Please help/explain.