No need to panic any longer! Google has confirmed that the star reviews in rich snippets, found in SERPs, were only missing because of a bug. There don't appear to be any plans to remove them any time soon.
A story in The SEM Post has the details. "Mozcast reported stars dropped from about 30% of search results down to 20%," so the story goes. It was a significant drop and had many webmasters concerned.
According to John Mueller, "...we double checked that with the team...just to make sure because usually it's kind of normal that the structured data markup, the rich snippets kind of come and go depending on changes on our side."