Fixed for C#.

inverted test cases does explain the discrepancy in example provided and expected result. shouldn't this be fixed though?

Are you removing lowercase and UPPERCASE vowels?

"Expected" and "But was" are inverted becauset he test cases are using Assert.Equal() in a wrong way.
The expected value should be the first parameter, the returned value the second one.
Example:
Assert.AreEqual("Ths wbst s fr lsrs LL!", Kata.Disemvowel("This website is for losers LOL!");

If you use Assert.Equal() in this way you have the right "description" of the failure.

Description seems to contradict test cases - For example, the string "This website is for losers LOL!" would become "Ths wbst s fr lsrs LL!".
Test fails with the following:
Expected: "Ths wbst s fr lsrs LOL!"
But was: "Ths wbst s fr lsrs LL!"