Ok i have figured out the answer to this after doing some research, what you need to do is right click on the SmartPreviews.lrdata file (on a mac) and click on "Show Package Contents" you will then be able to see all of your smart previews with the correct time stamp! Not sure what the lrdata file doesn't update but good to know all the recent smart previews are there!
I think the explanation for the Previews.lrdata showing an updated modified date, while the Smart Previews.lrdata does not, is because folders (and the "package" operates like a regular folder, which is the way it appears under Windows) only show an updated modified date when root-level files within the folder are themselves updated. If you look at Previews.lrdata, you'll see two root-level files ("previews.db" and "root-pixels.db") which are database files which manage the constantly changing previews in the 0-F sub-folders. These two files are updated every time you use Lightroom, so the modified date is updated every time, and that reflects in the parent "folder" when you examine that.
Smart Previews.lrdata, OTOH, has no root-level files. Database files aren't apparently needed in the same way, presumably because there's not a lot of change in those sub-folders, just additions and deletions and no other churn, so I guess the name and location of an individual images's Smart Preview is stored in the catalog. As a result, there are no root-level files, so nothing to force a change of the package's modified date.