When using product layout, we will pick random products to collect reviews. This default behavior is generally used for previewing reviews collector page. In actual production, you should specify url parameters to tell us which products you want to collect reviews.
Use product_ids=PRODUCT_A_ID,PRODUCT_B_ID with comma-seperated ID values for list of products you wish to collect reviews.
https://reviews.example.com?product_ids=prd_wpgnildbgg50k5u,prd_d69ctxwe7cr52am
Alternatively, you can pass product_external_ids to retrieve products using product IDs from ecommerce platform.
https://reviews.example.com?product_external_ids=4169648570416,4510810767408