This bug can be experienced on the official documentation. When right-clicking the node to display the ContextMenuButtons, the click seems to only be registered when the text inside the button is clicked. When clicking left or right of the text, nothing happens at all.
Did you define the GraphObject.click behavior on the TextBlock or on the whole “ContextMenuButton”?
Thank you for pointing out a misfeature in that example, where you can see the click event is defined on the TextBlock.