My submission using islower and the same version of compiler got AC.

Google the cppreference for islower/isupper and check the output of islower('a')/isupper('A'). You'll be surprised how stupid it is.

islower and isupper are old C-style functions which return int with 0 for false and non-zero for true. So you had to write your code as any C programmer: !!islower(ch)

In fact I had the same problem due to this function and got a penalty. The fact is that this function does not return a bool result as we expected. Function islower() returns a non-zero value when it is a lowercase letter. You can check here for details.

You can try the below code in custom test to see:

#include <bits/stdc++.h>
using namespace std;

int main()
{
	ios::sync_with_stdio(0), cin.tie(0);
	cout << islower('A') << "\n"; // returns 0, it is not a lowercase letter
	cout << islower('a') << "\n"; // returns 2
	cout << islower('b') << "\n"; // returns 2
	cout << islower('z') << "\n"; // returns 2
	return 0;
}