Unexpected field in deserialization for projection expression with a positional operator

var find = collection.Find(x => x.ArrayField.Any(predicate));

var a = find.Project<TypeOfX>(

        Builders<TypeOfX>.Projection

            .Include(x => x.ArrayField[-1])

            .Exclude(x => x.Id)

    )

    .FirstOrDefault() // <-- OK

    ?.ArrayField[0];

var b = find.Project(x => x.ArrayField[-1])

    .FirstOrDefault(); // <-- MongoInternalException: Unexpected field