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I've been finding there are cases where selecting a face in the preview thumbnails will render the landmark preview in the thumbnail to the right of it.
One easy way to do this (my example is with a really clean source where every frame has a face):
Open up the manual tool with some video
Click on some face thumbnail. The landmarks are showing up on the correct thumb, and continue doing so if you hit Z/X to navigate.
Right-click on your thumbnail and do "delete face". No thumbnail is selected (since your current frame no longer has a face in the alignment data)
Press 'X' to move to the next face. The landmark preview shows up on the thumbnail to the RIGHT of the selected thumbnail.