Google said that as of December 14th and onwards that the reviews snippet rich results report in Google Search Console may show fewer review objects. This is because Google is no longer showing top-level rating objects in this report.
Google posted on December 14, 2021, “previously, top-level schema.org/Rating objects were counted as reviews in the Review Snippets rich results report. This is no longer the case. As a result, you may see a reduction in the number of Review objects in this report.”
So if you see a reduction in the number of reviews you see in this report, this is why.
Google likely no longer shows top-level schema.org/Rating objects as rich results in Google Search or never did and is adjusting the report to reflect that.
As a heads up, Google added this reviews rich report in February 2020.
Forum discussion at Twitter.