I'm not sure why this is happening. The first node in the tree won't let me click on and select it. The only way to get it to select is to either do it programatically or to click on another node in the tree first and then click the first node. I have HideSelection set to false. I can't think of any other reason for this behavior. I have many trees and all of them display this behavior. This is really problematic if there is only one node in the tree and it can't be selected.
Can you duplciate this behavior in a small sample project? If so, I recommend that you Submit an incident to Infragistics Developer Support so they can check it out.