Why is my for loop not registering the first line in it's block?

I have recently started learning the basics of coding from codeacademy and was given a project involving applying something called the "Luhn Algorithm" for validating credit card numbers. I think I understand the algorithm, but I seem to be getting something wrong in my javascript code.

I'm using this as a sample to test if it works

const invalid1 = [4, 5, 3, 2, 7, 7, 8, 7, 7, 1, 0, 9, 1, 7, 9, 5];

I used that Array for this piece of code:

const validateCred = arr => {
  for (let i = arr.length - 1; i >= 0; i-=2) {
    for (let j = arr.length - 2; j >= 0; j-=2) {
      arr[j] *= 2;
      if (arr[j] > 9) {
        arr[j] -= 9;
      }
      const redFunc = (tot,num) => {
        return tot + num;
      }
      return total = arr.reduce(redFunc,0);
      
    }
  }
}
console.log(validateCred(invalid1));

And the answer that logs is 82. I checked the math manually and found out that it should be 85.

I figured out that the problem is that this code isn't registering

arr[j] *= 2;

I've been at it for hours but can't, for the life of me, figure out how to fix it. Please help.