This is a known issue in Aperture 3.2.3. To work around the issue, resize the viewer window slightly which causes the image to update. Applying any adjustment to the image will also cause the viewer to update.
Aperture 3.2.3: Viewer does not update after editing an image in external editor
When you return to Aperture after sending an image to an external editor, the viewer does not update to show the edits applied to the image, although the thumbnail does show the updates.
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