Not a bug, but rather a content issue.
This unit is a good example of why NS should allow users to select from any of the example sentences to add to their reviews.
One of the sentences, 麒麟は国が平和な時にしか現れん, talks about a mythical creature that is pretty esoteric. Whereas one of the other example sentences (龍は権力の象徴です) is not only more relevant, but also reinforces two fairly complex vocabulary - 権力and 象徴, yet we can't add it to our reviews.