Why does Preview lose notes?

I just reopened a .pdf document which I had previously edited with Preview using the "edit" tool. While all my added text and lines are still visible, all notes that I added have become unreadable. I still see the yellow "stickies" icons where I put them, but nothing seems to be able to coax the information I spent considerable time entering back to visibility. Is there a trick that I'm missing, or is this just a bug in Preview (in which case it would be better to have this function disabled to avoid wasting one's time)?