I have a fix in for this; possibly going out next week. Had to add a release and dlc field to the endpoint because it turns out everyone reviews the releases of the game and never just the game itself.
@frobie: Is there any chance that this change affected game filtering for other resources? As of yesterday, I noticed that I can no longer filter the "releases" endpoint by game ID. I posted a more thorough explanation of this issue in response to another user having this problem here: