A confirmed Google Maps bug seems to be causing the reviews in Google Maps to disappear. A Google Maps Help thread has several business owners complaining that their reviews (all or many) are now missing.
Googler, Joel H said that Google is aware of the issue and is working to resolve it. I'll quote him:
We're aware of this issue and are working on a fix. We'll update the group when more information becomes available.
I am not sure if this is a specific issue with importing reviews from third-party data sources or if it is a Google Maps review bug. I know that when Google Maps began allowing user contributed reviews back in June 2007, I did have at least one review, from myself. Now, none.
Embedded Maps view (currently has no review, but might change in future):
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Forum discussion at Google Maps Help.