SshClient.Connect() 'MAC Error' exception

[nick-owens]

Before I dive into the details, is this the right place to raise the question? I've had my fingers burned elsewhere. Sincere apologies if not but please delete me gently and suggest a better path.
I am writing a command line client to connect to a new SSH implementation on an industrial PLC. The SSH server is still vary basic and may have issues, but I can connect with (Open)ssh in a Windows command window, but with SshClient, I get an exception 'MAC Error' when connecting. What is a MAC error? Is it an unsupported MAC? (the server does hmac-sha1-96,hmac-sha1,hmac-md5 but they could add some sha2's). Or was the MAC agreed and then something went wrong?
I can offer Wireshark captures with OpenSSH and my failing code, and a verbose OpenSSH log. And the code as well, there isn't much there yet. Visual Studio 2022, C#.