That's a problem with your code, not a kata issue, this code passes those two tests and fail the other obviously:

using System;
using System.Text.RegularExpressions;

public class MorseCodeDecoder
{
    public static string DecodeBits(string bits)
    {
        if (bits == "1") return ".";
        else return "..";
    }

    public static string DecodeMorse(string morseCode)
    {
        Console.WriteLine(morseCode);
        return MorseCode.Get(morseCode);
    }
}