I have been noticing a recent spike in the number of complaints about their business reviews going missing in Google Maps. One of those complaints has a response from a Googler.
A Google Maps Help thread has one business listing owner complaining that he lost 16 reviews. Googler, Brianna said that Google is looking into the issue, she said:
We are looking into this. When did you notice the occurrence of the disappearance of your reviews?
So this possibly might be a larger issue that the Google Maps team is working on resolving. If you lost reviews for your business, I would chime in at the Google Maps Forum.
Forum discussion at Google Maps Help.