Why does array.prototype.slice() with no arguments change my output?

I am doing this challenge on leetcode.

• Challenge:
  https://leetcode.com/problems/relative-ranks/submissions/

Why is it that, this answer is accepted?

My Code:

/**
 * @param {number[]} nums
 * @return {string[]}
 */
var findRelativeRanks = function(nums) {
    const map = new Map();
    let result;
    nums
        .slice()
        .sort((a, b) => b - a)
        .forEach((num, i) => map.set(num, (i + 1).toString()));
    result = nums.map(num => {
        switch(map.get(num)) {
            case "1":
                return "Gold Medal";
            case "2":
                return "Silver Medal";
            case "3":
                return "Bronze Medal";
            default:
                return map.get(num);
        }
    })
    return result;
};

But if I just remove .slice(), some test cases fail. Why?