For sure a nice Kata, but yes the 4 test case is bugged.
Get the same exception for testing purpose did catch the NullReferenceException, saying values differ at index 0,1.

Please consider completing the code within one function, do not call recursive.

This is the only reason that I can think of.

I've got the same exception on that test case. To get a better understanding of the problem I added the following to the beginning of my solution to simply print the array on screen.

int XX = array.GetLength(0);
int YY = array.GetLength(1);          
          
for (int i = 0; i<XX; i++)
{
for (int j = 0; j<YY; j++)
 Console.Write(array[i,j]);           
} Console.WriteLine("------");

For unknown reasons I pass every test with those lines added, while removing them results in the NullReferenceException.

paste your code here and mark it as having spoiler content.

My code works fine, perhaps your code has some problems ;-)