On 16th September 2019, Google announced to introduce another update, although it is in beta testing form as of now. It shall determine whether a particular review, i.e. the stars should appear in the search result snippets or not. (Source)

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Soon, with its ability to transform, Google guarantees that it shall ease the implementation of the said feature very soon.

To cut short of all the extra information you might be seeing, here’s what the review rich results will consider from now on:

  1. The display of schema types shall become limited, which as of now include

2. Markup shall require a “name” property – Basically, while putting up a review on Google, it has become a must to specify the name of the product that is being reviewed, for it to be shown on the review rich results.

3. Reviews that are self-serving will be avoided – Google will no longer consider reviews that have been posted about an entity on its own website through a 3rd party widget or directly via their markup. These may be false and are not beneficial to the user.

Here’s what Google said:
We call reviews “self-serving” when a review about entity A is placed on the website of entity A – either directly in their markup or via an embedded 3rd party widget.

References:
https://9to5google.com/2019/09/16/google-search-star-ratings/