I tried this using the latest version of Rebex FTP/SSL:
var ftp = new Ftp();
ftp.LogWriter = new ConsoleLogWriter(LogLevel.Debug);
ftp.Settings.SslServerName = "siteground.biz"; // works around misconfigured server certificate
ftp.Connect("ftp.dlptest.com", SslMode.Explicit);
ftp.Login("dlpuser@dlptest.com", "...");
ftp.PutFile(@"c:\data\myfile.txt", "myfile.txt");
ftp.SetFileDateTime("myfile.txt", DateTime.Now);
ftp.GetRawList();
The log shows that MFMT was in fact used:
2020-09-30 10:32:25.467 INFO Ftp(1)[1] Info: Connecting to ftp.dlptest.com:21 using Ftp.
2020-09-30 10:32:25.476 INFO Ftp(1)[1] Info: Assembly: Rebex.Ftp 2020 R3 for .NET Standard 2.1 (Trial)
2020-09-30 10:32:25.482 INFO Ftp(1)[1] Info: Platform: Windows 10.0.19041 64-bit; CLR: .NET Core 3.1.8
2020-09-30 10:32:25.487 INFO Ftp(1)[1] Info: Using proxy none.
2020-09-30 10:32:25.789 INFO Ftp(1)[1] Response: 220-#########################################################
2020-09-30 10:32:25.791 INFO Ftp(1)[1] Response: 220-Please upload your web files to the public_html directory.
2020-09-30 10:32:25.792 INFO Ftp(1)[1] Response: 220-Note that letters are case sensitive.
2020-09-30 10:32:25.792 INFO Ftp(1)[1] Response: 220-#########################################################
2020-09-30 10:32:25.793 INFO Ftp(1)[1] Response: 220 This is a private system - No anonymous login
2020-09-30 10:32:25.804 INFO Ftp(1)[1] Command: AUTH TLS
2020-09-30 10:32:25.925 INFO Ftp(1)[1] Response: 234 AUTH TLS OK.
2020-09-30 10:32:26.265 INFO Ftp(1)[1] TLS: Negotiating TLS 1.2, RSA with ephemeral ECDH, AES with 256-bit key in GCM mode, AEAD.
2020-09-30 10:32:26.555 INFO Ftp(1)[1] TLS: Connection secured using cipher: TLS 1.2, RSA with ephemeral ECDH, AES with 256-bit key in GCM mode, AEAD.
2020-09-30 10:32:26.561 INFO Ftp(1)[1] Command: USER dlpuser@dlptest.com
2020-09-30 10:32:26.684 INFO Ftp(1)[1] Response: 331 User dlpuser@dlptest.com OK. Password required
2020-09-30 10:32:26.687 INFO Ftp(1)[1] Command: PASS **********
2020-09-30 10:32:26.832 INFO Ftp(1)[1] Response: 230-Your bandwidth usage is restricted
2020-09-30 10:32:26.833 INFO Ftp(1)[1] Response: 230 OK. Current restricted directory is /
2020-09-30 10:32:26.835 INFO Ftp(1)[1] Command: FEAT
2020-09-30 10:32:26.955 INFO Ftp(1)[1] Response: 211-Extensions supported:
2020-09-30 10:32:26.957 INFO Ftp(1)[1] Response: EPRT
2020-09-30 10:32:26.958 INFO Ftp(1)[1] Response: IDLE
2020-09-30 10:32:26.958 INFO Ftp(1)[1] Response: MDTM
2020-09-30 10:32:26.959 INFO Ftp(1)[1] Response: SIZE
2020-09-30 10:32:26.959 INFO Ftp(1)[1] Response: MFMT
2020-09-30 10:32:26.959 INFO Ftp(1)[1] Response: REST STREAM
2020-09-30 10:32:26.960 INFO Ftp(1)[1] Response: MLST type*;size*;sizd*;modify*;UNIX.mode*;UNIX.uid*;UNIX.gid*;unique*;
2020-09-30 10:32:26.960 INFO Ftp(1)[1] Response: MLSD
2020-09-30 10:32:26.961 INFO Ftp(1)[1] Response: AUTH TLS
2020-09-30 10:32:26.962 INFO Ftp(1)[1] Response: PBSZ
2020-09-30 10:32:26.963 INFO Ftp(1)[1] Response: PROT
2020-09-30 10:32:26.964 INFO Ftp(1)[1] Response: UTF8
2020-09-30 10:32:26.965 INFO Ftp(1)[1] Response: TVFS
2020-09-30 10:32:26.966 INFO Ftp(1)[1] Response: ESTA
2020-09-30 10:32:26.966 INFO Ftp(1)[1] Response: PASV
2020-09-30 10:32:26.967 INFO Ftp(1)[1] Response: EPSV
2020-09-30 10:32:26.968 INFO Ftp(1)[1] Response: SPSV
2020-09-30 10:32:26.969 INFO Ftp(1)[1] Response: ESTP
2020-09-30 10:32:26.970 INFO Ftp(1)[1] Response: 211 End.
2020-09-30 10:32:26.979 INFO Ftp(1)[1] Command: OPTS UTF8 ON
2020-09-30 10:32:27.100 INFO Ftp(1)[1] Response: 200 OK, UTF-8 enabled
2020-09-30 10:32:27.104 INFO Ftp(1)[1] Command: OPTS MLST type;size;modify;UNIX.mode;
2020-09-30 10:32:27.225 INFO Ftp(1)[1] Response: 200 MLST OPTS type;size;sizd;modify;UNIX.mode;UNIX.uid;UNIX.gid;unique;
2020-09-30 10:32:27.249 INFO Ftp(1)[1] Command: PBSZ 0
2020-09-30 10:32:27.370 INFO Ftp(1)[1] Response: 200 PBSZ=0
2020-09-30 10:32:27.372 INFO Ftp(1)[1] Command: PROT P
2020-09-30 10:32:27.493 INFO Ftp(1)[1] Response: 200 Data protection level set to "private"
2020-09-30 10:32:27.498 INFO Ftp(1)[1] Command: TYPE I
2020-09-30 10:32:27.619 INFO Ftp(1)[1] Response: 200 TYPE is now 8-bit binary
2020-09-30 10:32:27.624 INFO Ftp(1)[1] Command: PASV
2020-09-30 10:32:27.746 INFO Ftp(1)[1] Response: 227 Entering Passive Mode (35,209,241,59,135,218)
2020-09-30 10:32:27.880 INFO Ftp(1)[1] Command: STOR myfile.txt
2020-09-30 10:32:28.001 INFO Ftp(1)[1] Response: 150 Accepted data connection
2020-09-30 10:32:28.008 INFO Ftp(1)[1] TLS: Data connection: Trying to resume session.
2020-09-30 10:32:28.131 INFO Ftp(1)[1] TLS: Data connection: Negotiating TLS 1.2, RSA with ephemeral ECDH, AES with 256-bit key in GCM mode, AEAD.
2020-09-30 10:32:28.133 INFO Ftp(1)[1] TLS: Data connection: Resuming session.
2020-09-30 10:32:28.135 INFO Ftp(1)[1] TLS: Data connection: Connection secured using cipher: TLS 1.2, RSA with ephemeral ECDH, AES with 256-bit key in GCM mode, AEAD.
2020-09-30 10:32:28.270 INFO Ftp(1)[1] Response: 226-File successfully transferred
2020-09-30 10:32:28.272 INFO Ftp(1)[1] Response: 226 0.014 seconds (measured here), 294.49 bytes per second
2020-09-30 10:32:28.277 INFO Ftp(1)[1] Command: MFMT 20200930083228 myfile.txt
2020-09-30 10:32:28.398 INFO Ftp(1)[1] Response: 213 UTIME OK
2020-09-30 10:32:28.406 INFO Ftp(1)[1] Command: TYPE A
2020-09-30 10:32:28.527 INFO Ftp(1)[1] Response: 200 TYPE is now ASCII
2020-09-30 10:32:28.530 INFO Ftp(1)[1] Command: PASV
2020-09-30 10:32:28.652 INFO Ftp(1)[1] Response: 227 Entering Passive Mode (35,209,241,59,135,63)
2020-09-30 10:32:28.779 INFO Ftp(1)[1] Command: LIST
2020-09-30 10:32:28.901 INFO Ftp(1)[1] Response: 150 Accepted data connection
2020-09-30 10:32:28.905 INFO Ftp(1)[1] TLS: Data connection: Trying to resume session.
2020-09-30 10:32:29.027 INFO Ftp(1)[1] TLS: Data connection: Negotiating TLS 1.2, RSA with ephemeral ECDH, AES with 256-bit key in GCM mode, AEAD.
2020-09-30 10:32:29.031 INFO Ftp(1)[1] TLS: Data connection: Resuming session.
2020-09-30 10:32:29.032 INFO Ftp(1)[1] TLS: Data connection: Connection secured using cipher: TLS 1.2, RSA with ephemeral ECDH, AES with 256-bit key in GCM mode, AEAD.
2020-09-30 10:32:29.441 INFO Ftp(1)[1] Response: 226-Options: -a -l
2020-09-30 10:32:29.442 INFO Ftp(1)[1] Response: 226 12 matches total
Could you try running the same code yourself?