Oh yes, I worked at a review generation company for 6 years and to some capacity ever since, and that's always going to be the case. But the review sites are very wary of filtering reviews so they are 5-star and they have no problem stripping down your reviews at any time with no explanation if they are even a little suspicious.
For example, if you're caught asking the user for a simple NPS, and if it's 9 or 10, then triggering a review request only for high scores? Oooooh Google and Yelp especially get mad.
So, it's a tricky conversation as their marketer because you can't say, “just always do good work!!” (Haha) but there are plenty of ways to handle bad reviews when they come in, they always will, and that's why volume is important. Respond directly to the bad review for the world watching online, then offer to resolve the issue directly.
Hope any of that is helpful! The target is always moving to get online reviews so it’s good to keep asking these questions!