It seems the image referenced in the image attribute of the treeNode items is being rendered in the wrong element. This causes incorrect behavior when the showButtonLabels attribute is set to false, hiding the images as well as the labels. The tag is being rendered inside a with a class of dijitButtonText, along with the label text, and leaving empty a with the class of dijitIcon.

In the xpages demo, the Core_Outline page works around this on the Icon Toolbar with Labels by using classes and CSS styles instead of the intended functionality. I could do the same, but it seems a straightforward fix to get this to work as designed.