Google Kickstart - Wrong answer if cout.clear() is not used - c++

Question

I find this very stupid to ask, but I've been trying a question on Google Kickstart (Round A, 2021).
Now, firstly, I'd like to clarify that I do NOT need help with the question itself, but I'm encountering a weird issue that seems to be compiler-related. This problem only arises on certain compilers.
I am posting the question link, then the question statement if someone does not wish to use the link, then the problem I'm facing along with the code that works and the code that doesn't work.
Question Title: K-Goodness String, Round A (2021)
Question Link: https://codingcompetitions.withgoogle.com/kickstart/round/0000000000436140/000000000068cca3
Problem
Charles defines the goodness score of a string as the number
of indices i such that Si ≠ SN−i+1 where 1≤i≤N/2 (1-indexed). For
example, the string CABABC has a goodness score of 2 since S2 ≠ S5 and
S3 ≠ S4.
Charles gave Ada a string S of length N, consisting of uppercase
letters and asked her to convert it into a string with a goodness
score of K. In one operation, Ada can change any character in the
string to any uppercase letter. Could you help Ada find the minimum
number of operations required to transform the given string into a
string with goodness score equal to K?
Input
The first line of the input gives the number of test cases, T. T
test cases follow.
The first line of each test case contains two integers N and K. The
second line of each test case contains a string S of length N,
consisting of uppercase letters.
Output
For each test case, output one line containing Case #x: y,
where x is the test case number (starting from 1) and y is the minimum
number of operations required to transform the given string S into a
string with goodness score equal to K.
Sample Input:
2
5 1
ABCAA
4 2
ABAA
Sample Output:
Case #1: 0
Case #2: 1
Explanation:
In Sample Case #1, the given string already has a goodness score of 1. Therefore the minimum number of operations required is 0.
In Sample Case #2, one option is to change the character at index 1 to B in order to have a goodness score of 2. Therefore, the minimum number of operations required is 1.
The issue:
The problem is fairly straightforward, however, I seem to be getting a wrong answer in a very specific condition, and this problem only arises on certain compilers, and some compilers give the correct answer for the exact same code and test cases.
The specific test case:
2
96 10
KVSNDVJFYBNRQPKTHPMMTZBHQPZYQHEEEQFQWOJHPHFBFXGFFGXFBFHPHJOWQFQEEEHQYZPQHBZTMMPHTKPQRNBYFFVDNXIX
95 7
CNMYPKORAUTSYETNXAZQZGBFSJJNMOMINYKNTMHTARUMDXAJAXDMURATHMTNKYNIMOMNJJSFBGZQZAXNTEYSTUAROKPKJCD
Expected Output:
Case #1: 6
Case #2: 3
The problem arises when I do NOT use std::cout.clear() at a very specific place in my code. Just printing the value of any random variable also seems to solve this issue, it doesn't necessarily have to be cout.clear() only. I'm pasting the codes below.
**Original Code (Gives incorrect answer):**
//
// main.cpp
// Google Kickstart - Round A (2021)
//
// Created by Harshit Jindal on 10/07/21.
//
#include <iostream>
#define endl "\n"
using namespace std;
int main() {
int num_test_cases;
cin >> num_test_cases;
for (int test_case = 1; test_case <= num_test_cases; test_case++) {
int answer = 0;
int N, K;
cin >> N >> K;
char s[N];
cin >> s;
int current_goodness = 0;
for (int i = 0; i < N/2; i++) {
if (s[i] != s[N-1-i]) { current_goodness++; }
}
answer = abs(current_goodness - K);
cout << "Case #" << test_case << ": " << answer << endl;
}
return 0;
}
Incorrect Result for original code:
Case #1: 6
Case #2: 6
Modified Code (With cout.clear() which gives correct answer):
//
// main.cpp
// Google Kickstart - Round A (2021)
//
// Created by Harshit Jindal on 10/07/21.
//
#include <iostream>
#define endl "\n"
using namespace std;
int main() {
int num_test_cases;
cin >> num_test_cases;
for (int test_case = 1; test_case <= num_test_cases; test_case++) {
int answer = 0;
int N, K;
cin >> N >> K;
char s[N];
cin >> s;
int current_goodness = 0;
for (int i = 0; i < N/2; i++) {
if (s[i] != s[N-1-i]) {
current_goodness++;
}
cout.clear();
}
answer = abs(current_goodness - K);
cout << "Case #" << test_case << ": " << answer << endl;
}
return 0;
}
Correct Result for modified code:
Case #1: 6
Case #2: 3
A few additional details:
This issue is NOT coming up on my local machine, but on Google Kickstart's Judge with C++17 (G++).
Answer for Case #2 should be 3, and NOT 6.
This issue does NOT come up if only the second test case is executed directly, but only if executed AFTER test case #1.
The issue is ONLY resolved if the cout.clear() is placed within the for loop, and nowhere else.
We don't necessarily have to use cout.clear(), any cout statement seems to fix the issue.
I know it's a long question, but given that a problem is only coming up on certain machines, I believe it would require a deep understanding of c++ to be able to understand why this is happening, and hence posting it here. I'm curious to understand the reasoning behind such a thing.
Any help is appreciated.

Answer 1

As pointed out by Paddy, Sam and Igor in the comments, here is the solution as I understand it:
The problem arises because char s[N] is NOT C++ standard, any variable length arrays, for that matter. That might cause a buffer overrun, and will write over memory outside of the array, causing all sorts of weird behaviour.
The best way to avoid these kinds of bugs is to make it logically impossible for them to happen. – Sam Varshavchik
In this case, using string s solved the issue without having to call cout.clear().
Also, using #define endl "\n" might be faster when redirecting output to files, but since we're importing the entire std namespace, any person who does std::cout will get an error because it'll essentially get translated to std::"\n" which does not make sense.