For each zoom setting from 20% to 110%, the horizontal grid lines do not show up for me, only the vertical lines.
I'm using Google Chrome 18.0.1025.162, on Windows Vista, with Flash 11.2.202.
I've tried reopening the campaign in new tabs, as well as creating new pages in the campaign, and the problem persists.
The lines do show properly on all zoom levels using Firefox, though.