The visualizer has no idea of what property of a node to use for the caption, and sometimes it doesn't select anything for the caption.
You can change what is used for the caption on a label-wide basis.
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Click on the node label (near the top, just underneath the query string). When you do, you will see at the bottom of the result pane additional controls for changing how nodes of that label will display, including color, size, and caption to use.
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Click on the desired property to use as the caption.