Visual Studio 2017 C++Debug/Release different output

I've implemented a Kolakoski's sequence with a low memory footprint, using the reference from Wikipedia

#include <iostream>
#include <iomanip>
#include <vector>

int IncrementPointer(std::vector<int>& vec, int k)
{
    if (vec.size() <= k)
    {
        vec.push_back(22);
    }

    if (vec[k] == 11)
    {
        vec[k] = 1;
        return 1;
    }
    else if (vec[k] == 22)
    {
        vec[k] = 2;
        return 2;
    }
    else if (vec[k] == 1)
    {
        vec[k] = IncrementPointer(vec, k + 1) == 1 ? 2 : 22;
        return 2;
    }
    else if (vec[k] == 2)
    {
        vec[k] = IncrementPointer(vec, k + 1) == 1 ? 1 : 11;
        return 1;
    }

    return 0;
}

int main()
{
    long long iteration = 2;
    long long nextPowOf10 = 10;

    long long numOf1s = 1;

    std::vector<int> vec;

    std::cout << std::setw(15) << 'n' << std::setw(15) << "#1s" << std::setw(8) << "P(n)\n";
    std::cout << std::setw(15) << 1 << std::setw(15) << numOf1s << '\n';

    while (iteration++ <= 100'000'000'000'000)
    {
        int retvalue = IncrementPointer(vec, 0);
        if (retvalue == 1)
            ++numOf1s;
        if (iteration % nextPowOf10 == 0)
        {
            std::cout << std::setw(15) << nextPowOf10 << std::setw(15) << numOf1s << std::setw(8) << vec.size() << '\n';
            nextPowOf10 *= 10;
        }
    }
    return 0;
}

Now, the program internally calculates right elements of the sequence in Debug Mode and outputs expected results. So far, so good.

The problem starts in Release mode, vector gets optimized away (how could it be?), and the elements calculated are now wrong.

The expected sequence is [[1 2] 2 1 1 2 1 2 2 etc.], with first two are preset. and in release mode the elements are [1 2] 2 1 1 1 1 1 2 ... Clearly, something wrong went on. And subsequently the output is unexpected, and the program crashes, with calling to malloc (so it does have somewhere vector reallocated).

What am I doing wrong? Is it simultaneous push_back to vector and update to the element of vector?