It's last line - return FirstToUpper. Your result sends to function and first symbol always will be upper :)

I can confirm, it's fairly easy to construct a board that should fail, but passes this test.

var rows = new int[][]
{
new int[] {4, 4, 4, 6, 7, 8, 9, 1, 2},
new int[] {6, 7, 2, 1, 9, 5, 3, 4, 8},
new int[] {1, 9, 8, 3, 4, 2, 5, 6, 7},
new int[] {8, 5, 9, 7, 6, 1, 4, 2, 3},
new int[] {4, 2, 6, 8, 5, 3, 7, 9, 1},
new int[] {7, 1, 3, 9, 2, 4, 8, 5, 6},
new int[] {9, 6, 1, 5, 3, 7, 2, 8, 4},
new int[] {2, 8, 7, 4, 1, 9, 6, 3, 5},
new int[] {4, 3, 5, 2, 8, 6, 1, 7, 9},
};

The LINQ query for square nodes is a good move, but checking the sum equal to 45 is not applicable to similar tasks. Here is a test that your algorithm does not perform, you can check.

new object[] //All rows,colums and nodes have sum 45 from 1+2+3+4+5+6+7+8+9
{
"Try again!",
new int[][]
{
new int[] {1,2,2,2,2,9,9,9,9},
new int[] {2,9,9,2,1,9,2,2,9},
new int[] {2,9,9,2,9,9,2,2,1},
new int[] {9,2,2,9,9,9,1,2,2},
new int[] {9,1,2,9,2,2,2,9,9},
new int[] {9,2,9,1,2,2,2,9,9},
new int[] {2,9,1,9,9,2,9,2,2},
new int[] {2,9,2,9,2,1,9,9,2},
new int[] {9,2,9,2,9,2,9,1,2},
},
},

The first word should always be with the upper symbol, I do not see something here as it is controlled, do not tell me?