Got a reply from technical support:
Snap to Grid was never really fully developed for the Staff View. There are some "snap-like" tendencies that tend to vary, but as you have noticed the behavior is unpredictable in SONAR X1. Snap to Grid is more intended for moving data around in the Track View and the Piano Roll View.
Other users have also reported to us that notes do not "Snap" consistently in the Staff View. We have created official reports regarding this internally, however at this time it is more of a feature request for enhancements moving forwards. In other words, Tech Support does not have any "Fixes" for you as Staff View specific snap behavior would have to be implemented in an update to the application or a new version of the application.
The only recommendation I can make is to consider moving notes in the Piano Roll View. SONAR X1c did introduce "Piano Roll View Specific Snap" settings again, so you can easily use this to snap notes to different intervals. I know its not quite the behavior you are looking for, as it requires working in a different view, but you would be able to move data around easily this way. You can find more details on PRV Specific Snap in X1c at
http://www.cakewalk.com/Support/kb/reader.aspx/2007013258