I can't see anything wrong with your code.  I just checked: the following program works fine:
[DelimitedRecord(",")]
public class MyClass
{
    public string Field1;
    [FieldQuoted('"', QuoteMode.OptionalForBoth, MultilineMode.AllowForRead)]
    public string ExchangeRate;
    public string Field3;
}
class Program
{
    static void Main(string[] args)
    {
        var engine = new FileHelperEngine<MyClass>();
        string fileAsString = @"a,b,c" + Environment.NewLine +
                              @"a,b,c" + Environment.NewLine + 
                              @"a,""b,c"",d";
        MyClass[] validRecords = engine.ReadString(fileAsString);
        // Check the ExchangeRate values for rows 0, 1, 2 are as expected
        Assert.AreEqual("b", validRecords[0].ExchangeRate);
        Assert.AreEqual("b", validRecords[1].ExchangeRate);
        Assert.AreEqual("b,c", validRecords[2].ExchangeRate);
        Console.ReadKey();
    }
}