Hi
Thanks. Yeah, the thing here is not just a rank decline, it is the page dropped from the index altogether. Google instead choosing a different page from our site to rank for the same terms, but at a lesser position. Confirmed page dropped using site: www.domain.com searches. This is an ecommerce site with some 5,000 distinct product pages. We are not new, we've been around with an ecommerce site since 1997. The current site has existed in current for ~14 years. Again, it's seemingly random product detail pages being dropped. No manual action, no site errors (500 etc), no redirects or other problematic factors.
Roughly 1/3 of all tracked keywords in our long-running Moz pro campaign experienced either rank declines and a different page from the site being returned for the search, or suddenly not ranking whatsoever due to this matter of pages disappearing from the index. We are talking about product detail pages with >100 customer reviews and other UCG, tons of backlinks etc.
Interestingly, a handful of pages and terms seem to have corrected overnight. Not all - but a handful. We updated sitemaps yesterday and added some sketchy domains to our disavow file last night. None of the "usual suspects" types of issues explain this phenomenon.