Since MongoDB doesn't support predicates inside the $first and $last operators, we should throw a not supported exception.

var list = await collection.Aggregate()
    .Group(x => x.Name, g => new { Key = g.Key, FavoritePet = g.Pets.First(x => x.Name == "Fluffy") })
    .ToListAsync();

Currently, I believe the predicates are getting dropped when we rewrite First/Last calls