I did absolutely nothing on the C# version and all tests passed. I did the attempt and all tests passed. I did not submit it obviously.

With preloaded code, you don't have to do anything special.

However, there is a problem with the instruction MorseCode.Get('.--'). This call results in an error(error CS1012: Too many characters in character literal)
To fix, use double quotes instead.

   MorseCode.Get(".--");

Which will return a "W"

I agree with Markus.
I would recommend adding a sample test with n=25 and m=200. This will result in a condition where m is exactly equal to a multiple of n. Without it, the final test will sometimes pass and sometimes fail because the random test does not always include a scenario where m is an exact multiple of n.

Assert.AreEqual(700, Kata.SumMul(25, 200));

I think that change will eliminate the confusion people are encountering.