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Just received confirmation via email from my adwords support rep that the QS drop is, in fact, a bug they are working now to resolve.
I can confirm that this issue did not impact serving (i.e., the system has been correctly computing auction-time Quality Score this entire time for ranking and pricing purposes).
This issue was limited to the reporting of Quality Score, and the fix should be live for all within the next 24 hours. For those keywords with a status of Low search volume, the fix should be live within the next few days.
Thank you all for your patience as we resolve this.
I'm also seeing a sharp rise in the First Page bid across my account.