I'm baffled. I can't seem to replicate the problem you're describing. The only thing I can think of is you might have a duplicate shape beneath the shape you're trying to edit. Grab the pointer tool, click outside the canvas so nothing is selected. Then click and drag your object to the side and see if there's anything below it. If there is, what's probably happening is there was a duplicate object below it. When you edit the node, it reveals that duplicate object. When you double click to add a new node, you might be double clicking the other object (which will select the other object instead of adding a new node).