Off the top of my head I beleive PRPSheet updates the variable, in this case revision, with the revision of the Model used in the drawing sheet. That would require and check out, and update to show properly in edrawings preview.
If you change it to PRP (without sheet) I think you'll get what you're looking for.
But this did not work either. I've tried to map it several ways with no luck.
Have you linked the field on the title block to the custom property of the part / assembly ?
Hope this helps !