FtpWebRequest not working correct on Xamarin.Mac?

I have a cross-platform project targeting Winforms and Xamarin.Mac sharing code. It uses System.Net.FtpWebRequest to list files on a remote device running a ftp-server. It works fine for the Winform-project but not as expected on the Mac side:

Listing the content of a directory on the ftp-server:

var ftprequest= new FtpWebRequest.Create("ftp://192.168.188.21:1234/QVM_Data");
ftprequest.Method =Ftp.ListDirectoryDetails;

From Winforms the ftp-server receives this commands:

[0:] USER QvX
[0:] PASS
[0:] OPTS utf8 on
[0:] PWD
[0:] TYPE I
[0:] PASV
[0:] LIST QVM_Data
[0:] QUIT

and the listing of folder QVM_Data is properly returned.

On the Mac-side, the server receives this commands:

[0:] USER QvX
[0:] PASS
[0:] OPTS utf8 on
[0:] PWD
[0:] CWD /
[0:] TYPE I
[0:] PASV
[0:] LIST
[0:] QUIT

which is different, specially the pathname, which must be listed, is missing.

Any ideas?