System clock not synchronized with NTP server using systemd-timesyncd

I followed [this answer here](https://unix.stackexchange.com/a/666973) , but it seems that my system clock doesn't synchronize with NTP server:

$ cat /etc/debian_version
10.9
$ egrep -v "^$|^#" /etc/systemd/timesyncd.conf
[Time]
NTP=x.y.z.t1
FallbackNTP=x.y.z.t2
$ sudo timedatectl set-ntp true
$ sudo systemctl restart systemd-timesyncd
$ systemctl status systemd-timesyncd
● systemd-timesyncd.service - Network Time Synchronization
   Loaded: loaded (/lib/systemd/system/systemd-timesyncd.service; enabled; vendor preset: enabled)
  Drop-In: /usr/lib/systemd/system/systemd-timesyncd.service.d
           └─disable-with-time-daemon.conf
   Active: active (running) since Wed 2022-08-24 16:46:29 CEST; 2ms ago
     Docs: man:systemd-timesyncd.service(8)
 Main PID: 23412 (systemd-timesyn)
   Status: "Idle."
    Tasks: 2 (limit: 4915)
   Memory: 1.4M
   CGroup: /system.slice/systemd-timesyncd.service
           └─23412 /lib/systemd/systemd-timesyncd

Aug 24 16:46:29 EncoderBack systemd: Starting Network Time Synchronization...
Aug 24 16:46:29 EncoderBack systemd: Started Network Time Synchronization.
$ timedatectl timesync-status
       Server: x.y.z.t1 (x.y.z.t1)
Poll interval: 1min 4s (min: 32s; max 34min 8s)
 Packet count: 0
$ timedatectl show-timesync
SystemNTPServers=x.y.z.t1
FallbackNTPServers=x.y.z.t2
ServerName=x.y.z.t1
ServerAddress=x.y.z.t1
RootDistanceMaxUSec=5s
PollIntervalMinUSec=32s
PollIntervalMaxUSec=34min 8s
PollIntervalUSec=1min 4s
Frequency=0
$ journalctl -u systemd-timesyncd.service -n 5
-- Logs begin at Mon 2022-08-22 15:20:05 CEST, end at Wed 2022-08-24 16:46:29 CEST. --
Aug 24 16:46:29 EncoderBack systemd: Stopping Network Time Synchronization...
Aug 24 16:46:29 EncoderBack systemd: systemd-timesyncd.service: Succeeded.
Aug 24 16:46:29 EncoderBack systemd: Stopped Network Time Synchronization.
Aug 24 16:46:29 EncoderBack systemd: Starting Network Time Synchronization...
Aug 24 16:46:29 EncoderBack systemd: Started Network Time Synchronization.
$ timedatectl status
               Local time: Wed 2022-08-24 16:46:29 CEST
           Universal time: Wed 2022-08-24 14:46:29 UTC
                 RTC time: Wed 2022-08-24 14:46:19
                Time zone: Europe/Paris (CEST, +0200)
System clock synchronized: no
              NTP service: active
          RTC in local TZ: no
$

EDIT0 : Here is a [tcpdump](https://www.tcpdump.org/manpages/tcpdump.1.html) trace while restarting systemd-timesyncd.service :

$ sudo tcpdump -v dst port 123
tcpdump: listening on eno1, link-type EN10MB (Ethernet), capture size 262144 bytes
16:46:34.136278 IP (tos 0x10, ttl 64, id 18841, offset 0, flags [DF], proto UDP (17), length 76)
    ntpclient.lan.53695 > ntpserver.lan.ntp: NTPv4, length 48
        Client, Leap indicator:  (0), Stratum 0 (unspecified), poll 0 (1s), precision 0
        Root Delay: 0.000000, Root dispersion: 0.000000, Reference-ID: (unspec)
          Reference Timestamp:  0.000000000
          Originator Timestamp: 0.000000000
          Receive Timestamp:    0.000000000
          Transmit Timestamp:   3870427594.031728329 (2022/08/25 16:46:34)
            Originator - Receive Timestamp:  0.000000000
            Originator - Transmit Timestamp: 3870427594.031728329 (2022/08/25 16:46:34)
^C
1 packet captured
1 packet received by filter
0 packets dropped by kernel

EDIT1 : Here is a [tshark](https://www.wireshark.org/docs/man-pages/tshark.html) trace while restarting systemd-timesyncd.service :

$ sudo tshark -n -f 'udp port 123' -c2
Running as user "root" and group "root". This could be dangerous.
Capturing on 'eno1'
    1 0.000000000 a.b.c.d  → x.y.z.t1 NTP 90 NTP Version 4, client
    2 0.000678872 x.y.z.t1 → a.b.c.d  NTP 90 NTP Version 3, server
C2 packets captured

EDIT2 : Thanks to @Bib and to the tshark output, it seems the systemd-timesyncd client sends NTPv4 protocol requests but the server responds with NTPv3 protocol answers. As @QuartzCristal and @Bib suggest, I will be using ntpsec. EDIT3: After having configured the /etc/ntpsec/ntp.conf file and restarted the ntpsec service, it works fine now :

$ grep ^server /etc/ntpsec/ntp.conf
server x.y.z.t1 iburst
server x.y.z.t2 iburst
$ sudo mkdir /var/log/ntpsec/
$ sudo chown ntpsec:ntpsec /var/log/ntpsec/
$ sudo systemctl restart ntpsec
$ systemctl status ntpsec.service
● ntpsec.service - Network Time Service
   Loaded: loaded (/lib/systemd/system/ntpsec.service; enabled; vendor preset: enabled)
   Active: active (running) since Fri 2022-08-26 11:06:49 CEST; 2s ago
     Docs: man:ntpd(8)
  Process: 22622 ExecStart=/usr/lib/ntp/ntp-systemd-wrapper (code=exited, status=0/SUCCESS)
 Main PID: 22625 (ntpd)
    Tasks: 1 (limit: 4915)
   Memory: 1.6M
   CGroup: /system.slice/ntpsec.service
           └─22625 /usr/sbin/ntpd -p /run/ntpd.pid -c /etc/ntpsec/ntp.conf -g -N -u ntpsec:ntpsec

Aug 26 11:06:49 EncoderBack ntpd: CLOCK: leapsecond file ('/usr/share/zoneinfo/leap-seconds.list'): loaded, expire=2021-12-28T00:00Z last=2017-01-01T00:00Z ofs=37
Aug 26 11:06:49 EncoderBack ntpd: CLOCK: leapsecond file ('/usr/share/zoneinfo/leap-seconds.list'): expired less than 242 days ago
Aug 26 11:06:49 EncoderBack ntpd: INIT: Using SO_TIMESTAMPNS
Aug 26 11:06:49 EncoderBack ntpd: IO: Listen and drop on 0 v6wildcard [::]:123
Aug 26 11:06:49 EncoderBack ntpd: IO: Listen and drop on 1 v4wildcard 0.0.0.0:123
Aug 26 11:06:49 EncoderBack ntpd: IO: Listen normally on 2 lo 127.0.0.1:123
Aug 26 11:06:49 EncoderBack ntpd: IO: Listen normally on 3 eno1 a.b.c.d:123
Aug 26 11:06:49 EncoderBack ntpd: IO: Listen normally on 4 lo [::1]:123
Aug 26 11:06:49 EncoderBack ntpd: IO: Listen normally on 5 eno1 [fe80::3e7c:3fff:fed4:a223%2]:123
Aug 26 11:06:49 EncoderBack ntpd: IO: Listening on routing socket on fd #22 for interface updates

Now the system clock is synchronized :

$ timedatectl
               Local time: Fri 2022-08-26 11:08:05 CEST
           Universal time: Fri 2022-08-26 09:08:05 UTC
                 RTC time: Fri 2022-08-26 09:08:05
                Time zone: Europe/Paris (CEST, +0200)
System clock synchronized: yes
              NTP service: n/a
          RTC in local TZ: no

EDIT4 : Here is a tcpdump output of what is going on when using ntpsec, the source packet tos has changed and the source port is now 123 :

$ sudo tcpdump dst port 123 -n -c 2 -v
tcpdump: listening on eno1, link-type EN10MB (Ethernet), capture size 262144 bytes
11:53:49.185280 IP (tos 0xb8, ttl 64, id 54505, offset 0, flags [DF], proto UDP (17), length 76)
    a.b.c.d.123 > x.y.z.t1: NTPv4, length 48
        Client, Leap indicator:  (0), Stratum 0 (unspecified), poll 0 (1s), precision 32
        Root Delay: 0.000000, Root dispersion: 0.000000, Reference-ID: (unspec)
          Reference Timestamp:  0.000000000
          Originator Timestamp: 0.000000000
          Receive Timestamp:    0.000000000
          Transmit Timestamp:   1839874488.898661747 (2094/05/28 04:43:04)
            Originator - Receive Timestamp:  0.000000000
            Originator - Transmit Timestamp: 1839874488.898661747 (2094/05/28 04:43:04)
11:53:49.185929 IP (tos 0x0, ttl 126, id 18818, offset 0, flags [none], proto UDP (17), length 76)
    x.y.z.t1.123 > a.b.c.d.123: NTPv3, length 48
        Server, Leap indicator:  (0), Stratum 1 (primary reference), poll 0 (1s), precision -23
        Root Delay: 0.000000, Root dispersion: 10.751129, Reference-ID: LOCL
          Reference Timestamp:  3870431575.277677199 (2022/08/25 17:52:55)
          Originator Timestamp: 1839874488.898661747 (2094/05/28 04:43:04)
          Receive Timestamp:    3870496473.230674199 (2022/08/26 11:54:33)
          Transmit Timestamp:   3870496473.230678499 (2022/08/26 11:54:33)
            Originator - Receive Timestamp:  +2030621984.332012452
            Originator - Transmit Timestamp: +2030621984.332016752
2 packets captured
2 packets received by filter
0 packets dropped by kernel

And here is a tshark output of what is going on when using ntpsec, the weird is that it is the same output as the one I got from using systemd-timesyncd.service (except the source port is now 123) :

$ sudo tshark -f 'udp port 123' -n -c 2
Running as user "root" and group "root". This could be dangerous.
Capturing on 'eno1'
    1 0.000000000 a.b.c.d  → x.y.z.t1 NTP 90 NTP Version 4, client
    2 0.000787978 x.y.z.t1 → a.b.c.d  NTP 90 NTP Version 3, server
2 packets captured