Can't send message to specific user with SignalR

I can't make works the message sending to one specific user from the code behind. Clients.All works, Clients.AllExcept(userId) works, but not Client.User(userId).

My hub:

public class MessagingHub : Hub
{
    public override Task OnConnected()
    {
        var signalRConnectionId = Context.ConnectionId;
        // for testing purpose, I collect the userId from the VS Debug window
        System.Diagnostics.Debug.WriteLine("OnConnected --> " + signalRConnectionId);
        return base.OnConnected();
    }
}

My controller to send message from code behind:

public void PostMessageToUser(string ConnectionId)
{
    var mappingHub = GlobalHost.ConnectionManager.GetHubContext<MessagingHub>();

    // doesn't works
    mappingHub.Clients.User(ConnectionId).onMessageRecorded();

    // doesn't works
    mappingHub.Clients.Users(new List<string>() { ConnectionId }).onMessageRecorded();

    // works
    mappingHub.Clients.All.onMessageRecorded();

    // works (?!)
    mappingHub.Clients.AllExcept(ConnectionId).onMessageRecorded();

}

How my hub is initialized on the JS:

var con, hub;
function StartRealtimeMessaging()
{
    con = $.hubConnection();
    hub = con.createHubProxy('MessagingHub');

    hub.on('onMessageRecorded', function () {
        $(".MessageContainer").append("<div>I've received a message!!</div>");
    });
    con.start();
}

And finally how I send a(n empty) message to the hub:

function TestSendToUser(connectionId)
{
    $.ajax({
        url: '/Default/PostMessageToUser',
        type: "POST",
        data: { ConnectionId: connectionId},// contains the user I want to send the message to
    });
}

So, it works perfectly with mappingHub.Clients.All.onMessageRecorded(); but not with mappingHub.Clients.User(ConnectionId).onMessageRecorded(); or mappingHub.Clients.Users(new List<string>() { ConnectionId}).onMessageRecorded();.

But interestingly, it works with mappingHub.Clients.AllExcept(ConnectionId).onMessageRecorded(); : All users connected receive the message except the given userid, which means the userid is good, and the user is well identified. So, why Clients.User(ConnectionId) doesn't works?