It happened to me too. It says that the name is Petrov`s defense, but doesn`t mark the moves as book.
Game review - missing book moves for Petrov's defense?
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When I review games that I've played against Petrov's defense, Game Review treats 2...Nf6 as a non-book move! This has only started to happen recently, since around the time that chess.com overhauled its book move classification system. I think this might be a classification error that requires correction.