https://techcrunch.com/2019/10/30/a-week-long-ios-app-store-bug-wiped-out-over-20m-ratings/

An accidental sweep of the App Store removed over 20 million ratings from the most popular apps — including from well-known brands like Google, Microsoft, Starbucks, Hulu, Nike, and others — as well as from smaller developers. The issue began on October 23, 2019 and wasn’t resolved until yesterday, October 29. Apple hasn’t yet explained how such a sizable and impactful change to app ratings occurred.

This massive ratings drop was spotted by the mobile app insights platform, Appfigures.