Templating and OpenMP causes free(): double free detected in tcache 2

I've worked for a while to get my code to a minimal reproducible example and I think I have it. See the single main.cpp function below, compiled (on Linux) one of two ways:

1. In serial: g++ -O3 --std=c++17 -o test_rho.exe main.cpp
2. With OpenMP: g++ -O3 --std=c++17 -fopenmp -o test_rho.exe main.cpp

Without OpenMP the code runs fine. With OpenMP, I get the following error:

About to return 0
free(): double free detected in tcache 2

When using OpenMP, the following changes to main.cpp will make this error go away:

• Don't define Wfn() constructor. More specifically, don't invoke _chi.resize(120).
• Don't use private _chi in the OpenMP loop.
• Don't use templating (instead just use double on everything).

What is causing a double free here and why do each of the changes above fix the problem?

main.cpp:

#include <iostream>
#include <vector>

// Wfn class
template<typename real>
class Wfn {
    public:
        Wfn() { _chi.resize(120); }
        std::vector<real> rho(int nrhos);

    private:
        std::vector<real> _chi;
};

// Single Wfn function.
template<typename real>
std::vector<real> Wfn<real>::rho(int nrhos) {
    std::vector<real> rhos;
    rhos.resize(nrhos);
    #pragma omp parallel for private (_chi)
    for (size_t irho = 0; irho < nrhos; irho++) {
        rhos[irho] = (real)irho;
    }
    return rhos;
}

// Main
int main(int argc, char** argv) {
    Wfn<double> wfn;
    std::vector<double> rhovs;

    rhovs = wfn.rho(1000);

    std::cerr << "About to return 0" << std::endl;
    return 0;
}

Update:

In case this helps, the general reason this is happening is because wfn is going out of scope, but my original question(s) still remain(s). If main is changed to:

int main(int argc, char** argv) {
    {   
        Wfn<double> wfn;
        std::vector<double> rhovs;

        rhovs = wfn.rho(1000);
        std::cerr << "About to exit scope" << std::endl;
    }   

    std::cerr << "About to return 0" << std::endl;
    return 0;
}

Then the error is

About to exit scope
free(): double free detected in tcache 2