I don't normally reply to my own posts, but thought I should in this case....
I missed this when I searched before, but this problem has been around for a while, I guess. The newest thread in this linked discussion is November:
I've submitted a bug report to Apple.
Thanks. I'm running Aperture 3.4.3, so there's nothing to update. I suppose it's possible (but highly doubtful) that, when I migrated everything from my old iMac to this new one, a patch file or two didn't make the move.
If no one else jumps in here, I will try your suggestion and delete/reinstall Aperture to see if that corrects it.