//raise event
if (productReview.IsApproved)
_eventPublisher.Publish(new ProductReviewApprovedEvent(productReview));
But it is missing when Product Review update from admin. I think it should raise from admin site too.
Note: nop version 3.80
//raise event
if (productReview.IsApproved)
_eventPublisher.Publish(new ProductReviewApprovedEvent(productReview));