Not a kata issue then.

You should have rewritten the tests without changing the behaviour of the kata's code, something like this:

using NUnit.Framework;
using System;

[TestFixture]
public class IsStringUppercaseUnitTests
{
    [TestCase("A", true)]
    [TestCase("ALLUPPER", true)]
    [TestCase("ALLUPPER WITHSPACE", true)]
    [TestCase("a", false)]
    [TestCase("alllower", false)]
    [TestCase("alllower withspace", false)]
    [TestCase("PascalCase", false)]
    [TestCase("mIxEd BaG", false)]
    public void IsUpperCase_WhenCalled_ReturnsTrueIfParameterIsAllUppercase(string input, bool output)
    {
        // Arrange
        var s = input;

        // Act
        var result = s.IsUpperCase();

        // Assert
        Assert.That(result, Is.EqualTo(output));
    }
}

Can you say which languages use state and have this keyword?

Your code won't work, have you seen the initial code?

public static class StringExtensions
{
  public static bool IsUpperCase(this string text)
  {
    // your code here
  }
}

In the tests, your code is used like this: "some string".IsUpperCase(), not like this: IsUpperCase("some string")

This will invalidate all solutions as kata description is shared among languages