Getting "SshConnectionException: Client not connected" on RunCommand despite being inside "if (client.IsConnected)"

Here is my code:

private void connectToMacSSH()
{
    using (var client = new SshClient(sshIp,
                                        22,
                                        sshUsername,
                                        new PrivateKeyFile(rsaKeyFilePath,
                                        sshRsaPrivateKeyPassphrase)))
        {

            client.Connect();

            if (client.IsConnected)
            {
                new Thread(() =>
                {
                    client.RunCommand("caffeinate -u -t 2");
                }).Start();
            }
            else
            {
                ...
            }
        }
    }
}

This is how it's called (in a main method):

sshThread = new Thread(() => { connectToMacSSH(); });
sshThread.Start();

This code was working 100% fine just minutes ago. I have no idea what I changed, but for some reason, even though client.IsConnected returns true, running the client.RunCommand line will throw "SshConnectionException: Client not connected", every time.

The command itself executes fine on another SSH client, using all of the credentials and login details on another SSH client works fine too. I have physical access to the host I am trying to SSH into, it is on (logged in and screen on), connected on the right static local IP, and I can access it via other SSH clients.

I've rewritten the code about 3 times now with no success - I even tried removing the try catch block (around RunCommand) and the command itself from a new thread and running it synchronously instead - it had no effect.

All help widely appreciated, completely unsure of what to try next...

Someone else had this issue... only answer was a dead link https://stackoverflow.com/questions/25522894/client-not-connected-exception-with-ssh-net

Start with the supplied connectToMacSSH entry point and the RunCommand call, then inspect the SSH client connection lifecycle around the threaded and synchronous variants. Reproduce the reported exception using the provided credentials and command, and determine whether the behavior is a library defect or an issue-specific lifecycle condition before defining a regression test or documentation update.