I have noticed this problem after installing the last Aperture (3.2.3) update. It appears to be common to my Mac Pro and Mac Book Pro. The updated preview is visible only in "viewer" mode (menu / view / viewer). It's interesting, that updated preview appears in "viewer" mode on both displays (I use two monitors). And when I switch into "split view" the preview switch also and is not updated.
After relaunching Aperture previews are updated.
The thumbnail looks to be good (updated). This problem makes my work really uncomfortable.