High SERP volatility has been recorded in both Google web and local search results from April 23, 2021. The fluctuations in web search results were picked on April 23rd and 24th, while BrightLocal has picked up fluctuations in local search results from April 23rd to 25th, spanning over 3 days.
Google’s Danny Sullivan has confirmed the rollout of the product reviews update is complete on April 22, 2021. Since this high SERP volatility is seen on the days following the product reviews update, the volatility is likely due to the change in the quality standard set by the product reviews update. Google has not confirmed any other updates during this period.