Incompatibility between NTS in ntpsec 1.2.1 and 1.1.8?

I've been trying to run a NTS-enabled ntpsec server with ntpsec 1.2.1 (version from Ubuntu jammy). 'chrony4' and ntpsec 1.2.0 (from Debian bullseye) as a client both can speak NTS with that server just fine. As can another ntpsec 1.2.1 client. However, all Ubuntu 20.04 clients (that use version 1.1.8) can NOT. They manage to get the NTS-KE-part done just fine, they seem to receive their cookies and be happy about it.

 NTSc: Using TLSv1.3, TLS_AES_256_GCM_SHA384 (256)
 NTSc: Good ALPN from: [redacted]:4460
 NTSc: read 880 bytes
 NTSc: Got 8 cookies, length 104, aead=15.
 NTSc: NTS-KE req to [redacted]:4460 took 0.017 sec, OK

However, all the NTP-requests the clients send with these cookies are then rejected by the server. Each causes a Log entry on the server like

  EX-REQ: Count=3 Print=3, Score=1.487, M3 V4 from [redacted]:123, lng=664
  EX-REQ:  23000020 00000000 00000000 0000[....]

And of course, the clients get no reply.

What irritates me is that both 1.1.8 and 1.2.1 can talk to nts.ntp.se just fine, just not to each other.

Is the incompatibility between NTS in 1.1.8 and 1.2.1 known and expected? Is there any way to get current ntpsec to cooperate with the 1.1.8 client with NTS? Because Ubuntu 20.04 is a LTS-release, those 1.1.8 clients are going to be around for quite a while, and running a rather large public NTP server, we would like to support these.