bool inside struct changes value nonsensically when put in vector [on hold]

The program below creates random numbers between 20 and 60, checks if they are prime, creates an instance of the stuct p, saves both values as n (number) and b (is-prime). Then it saves the struct into a vector. When the b value of the last element in the vector is retrieved it has changed (to a random int), also, sometimes when I compile and run it suddenly stops responding. Why is this?

bool isPrimo(int i) {
    for(int f = 2; f*f <= i; ++f)
        if(i%f == 0)
            return false;
    return true;
}
struct p {
    int n;
    bool b;
};
int main(){
    int const CANT = 15;

    srand(time(NULL));
    vector<p> j;
    for(int i = 0; i < CANT; i++) {
        p ni;
        ni.n = 20 + rand()%61;
        ni.b = isPrimo(ni.n);
        j.push_back(ni);
        cout<<j.back().n<<' '<<j.back().b<<' '<<ni.b<<endl;
    }
}

I have cleaned the project and the compiler is minGW (essentials: binutils 2.26, GCC 5.3.0, mingw-w64 4.0.6) C++14, IDE is CodeBlocks.