freeCodeCamp: Wrong solution is accepted when submitted

Challenge Return Largest Numbers in Arrays has an issue. User Agent is: Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/55.0.2883.87 Safari/537.36. The code below uses map/reduce as suggested in the medium forum (I’ve reviewed the solutions over there) and is accepted when submitted.

The problem is that there is an assumption that in every subarray not all of the values are negatives and it is not shown in the description of the problem. That is, if one of the subarray contains only negatives values, the accepted solution below isn’t correct.

My code:

function largestOfFour(arr) {
  return arr.map(function(subArray) {
    return subArray.reduce(function(prev, curr) {
      return (curr > prev ? curr : prev);
    }, 0);
  });
}

largestOfFour([[4, 5, 1, 3], [13, 27, 18, 26], [32, 35, 37, 39], [1000, 1001, 857, 1]]);