Google is currently working on fixing a bug that prevents the product variant structured data from showing rating stars in the search. This bug was spotted by Jarno van Driel and Ryan Levering from Google confirmed it and said they will work on a fix soon.
Jarno posted on LinkedIn saying, “The new schemaorg ProductGroup triggers a price-range Rich Result. However, it does not seem to trigger any rating stars in the search results – even though aggregateRating is part of the recommended properties for ProductGroup.”
He also posted this other example:
Ryan Levering from Google wrote “This one is likely a bug, you shouldn’t be eligible for product carousel markup and the rich snippet. The rich snippet targets single product/group pages.” He then later confirmed the bug saying, “I can confirm this is a bug (and an important one to fix). We’ll look at getting a fix out so you can push out the ProductGroup.”
Forum discussion at LinkedIn.