g++-13 the "-isystem" flag seems to work incorrectly with optimization enabled

Question

File "include/header.hpp"

#include <iostream>

class Eka
{
        int x;
public:
        Eka() { std::cout << x;};
        Eka(int y) : x(y) {}
        int Sqr() const { if(x > 5) return x + 1;};
};

file "eka.cpp"

#include <header.hpp>

int main() {
        Eka e;

        int x = e.Sqr();
        int y {};

        std::cin >> y;
        std::cout << x + y;

        return 0;
}

Build command:

g++ src.cpp -ores -Wall -Wextra -Werror -O1 -isystem include

Result:

g++11 hides warnings from "system" headers: compilation is successful

g++13.2 does not hide warnings: compilation failed. With "-O0" everything works fine.

What's the problem ?

I expect the same behavior from different compiler versions regardless of whether the optimization flag is present.

I expect that warnings from the "system" headers will be suppressed when optimization is enabled and the "isystem" flag is used.