If you just run this code

int main(){
    vector<long long> v(100000,42);
    sort(v.begin(),v.end(),[](int a, int b){return a<=b;});
    for (int i=0;i<v.size();i++) cout << v[i] << " ";
    cout << '\n';
    return 0;
}

There are some incorrect values at the beginning of the array sometimes, looks like the implementation of the sorting function relies on the ordering being strict, otherwise it can be undefined. I stumbled upon this some time ago when using a custom comparison on a set. I think the way they establish whether two elements are equal is that !(a < b) && !(b < a) => a = b. This wouldn't work for your comparison — there could be something similar in sorting function.