I don't mean to spread any false information, even if just theorizing. Nor am I very much affected by the issue at hand; not in my wallet anyway. Looking at the averages, there's a brief 25% drop on November 10, but overall coverage on my main site has dropped only about 4-5%, with the decline starting at the end of October, and currently at ~94%. I do see some empty slots intermittently, which when filled always display the same ad, whereas the same slot on other pages tend to load a bigger variety of ads. This happens both on popular and impopular pages.
The reason it doesn't particularly affect my bottom line is that the coverage rates only seem to drop for countries where RPMs are low anyway. Most of my revenue comes from Western, English-speaking countries like the US, UK, Canada and Australia, for which the coverage rates have remained at a pretty steady 99+%. Comparing that to India, for example, which sends more traffic than Australia, I see lots of intermittent drops, with November 10 again as the sharpest one, showing a mere 35% coverage rate, which combined with similar patterns for other countries explains the 25% drop I saw in the average rate.
Whatever country they're from, they visit the same pages, so crawler access is not the issue there. Of course, my previous theory regarding changes in labeling of content wouldn't make much sense either. Geography seems to have something to do with it, for me at least.
Have you checked your coverage rates by country? Where's your traffic from mostly?
[edited by: robzilla at 7:41 pm (utc) on Jan 22, 2018]