As seen in the picture, the requirements of being a Great Hall should be met. The room has an average decor of ~120, with no tile falling below 20. It contains a full water cooler and is 40 tiles in size. According to the description, this should be a Great Hall, but the game still treats it as a Mess Hall. In this thread, someone suggested that the game only checks for it being a Great Hall if the recreational building was placed last. Saving and loading the game did indeed force a re-check and it is now calculated as a great hall.