The following image shows the browser with thumbs completely rendered for the folder. Notice a single image has been selected.

The next image below shows the results of pressing the middle mouse button (toggle full screen) on the image selected in the browser. Notice there are no thumbs in the thumbs strip. Toggling back and forth between the browser and full screen never shows thumbs in the full screen thumbstrip.

Some folders will display thumbs properly in the full screen thumbstrip, but most do not. There does not appear to be a regular pattern to this behavior. Sometimes toggling back and forth between the browser and full screen will cause the thumbs to appear. Sometimes not.