Javascript – Simplification of prefix notation

I am working on a Kattis problem, where I am supposed to take the input in prefix notation, simplify it and return it in prefix notation as well.
These are the examples of inputs and outputs:

Sample Input 1                    Sample Output 1
+ 3 4                             Case 1: 7
- x x                             Case 2: - x x
* - 6 + x -6 - - 9 6 * 0 c        Case 3: * - 6 + x -6 - 3 * 0 c

I have written this piece of code, and if I run it with this kind of input data, I get exactly the same output as stated above. Yet, I get wrong answer from Kattis.

What is it that I am doing wrong here?
It is frustrating since you get no debugging hints.

const readline = require('readline');

const rl = readline.createInterface({
    input: process.stdin,
    output: process.stdout
});

const operators = ['+', '-', '*', '/'];
const operatorsFunctions = {
    '+': (a, b) => a + b,
    '-': (a, b) => a - b,
    '*': (a, b) => a * b,
    '/': (a, b) => a / b,
};

let lineNumber = 0;

rl.on('line', (line) => {
    const mathExpression = line.split(' ');
    lineNumber += 1;
    let result = [];
    let stack = [];

    for (let i = mathExpression.length -1; i >= 0; i--) {
        if (!isNaN(mathExpression[i])) {
            stack.unshift(mathExpression[i]);
        } else if (operators.includes(mathExpression[i])){
            if (!stack.length) {
                result.unshift(mathExpression[i]);
            }
            if (stack.length === 1) {
                result.unshift(stack[0]);
                result.unshift(mathExpression[i]);
                stack = [];
            }
            if (stack.length > 1) {
                const sum = operatorsFunctions[mathExpression[i]](Number(stack[0]), Number(stack[1]))
                stack.splice(0, 2, sum);
                if (i === 0) {
                    result.unshift(...stack);
                }
            }
        } else {
            if (stack.length) {
                result.unshift(...stack);
                stack = [];
            }
            result.unshift(mathExpression[i]);
        }
    }
    const text = `Case ${lineNumber}: ${result.join(' ')}`;
    console.log(text);
});

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