In version 1.7.3, the native Array.from is overwritten by $A as below:

function $A(iterable) {
  if (!iterable) return [];
  if ('toArray' in Object(iterable)) return iterable.toArray();
  var length = iterable.length || 0, results = new Array(length);
  while (length--) results[length] = iterable[length];
  return results;
}

Array.from = $A;

From the definition of these two functions:
The Array.from() method creates a new Array instance from an array-like or iterable object.
However, the $A function only accepts an array-like collection (anything with numeric indices).
So $A is not able to handle other iterable objects without numeric indices such as MapIterator.

When $A try to create a new array by reading the iterable.length which does exist in some native iterable objects such as MapIterator, the result will be an array with length 0 returned back.

Hope this will be fixed soon.

Thanks,
Max