I've spotted a few posts in the forums complaining about how Yahoo is reporting very few clicks and impressions on both the Yahoo Search Marketing side and Yahoo Publisher Network side. The simple answer is that this may not a bug, but rather a reporting delay due to a maintenance update.
Yes, there is a lot of confusion about this update, since there was no mention of it on either the official YPN Blog or the YSM Blog, but there is a notification that an update is coming this coming weekend (not the past weekend). So there are rumors that Yahoo put the wrong date on the notification.
If you login to the YPN console, you will see a notification placed on the 21st that warns of an maintenance period on the 28th. Here is the message:
Notification of Scheduled Maintenance - Beginning 8/28/09
On Friday August 28, 2009 there will be a temporary interruption in the ability to access the Yahoo! Publisher Network portal. This comes as a part of planned system maintenance, and will take place on Friday Aug. 28, 2009 from 6:00 pm PDT to Saturday Aug. 29, 2009 2:00 am PDT. We apologize in advance for any inconvenience this may cause and appreciate your patience.
Again, there are rumors that the maintenance is going on now, which is why there is a reporting lag on both the YPN and YSM side.