Drain the entropy of std::random_device

I'm using std::random_device and would like to check for its remaining entropy. According to cppreference.com:

std::random_device::entropy

double entropy() const noexcept;

[...]

Return value

The value of the device entropy, or zero if not applicable.

Notes

This function is not fully implemented in some standard libraries. For example, LLVM libc++ always returns zero even though the device is non-deterministic. In comparison, Microsoft Visual C++ implementation always returns 32, and boost.random returns 10.

The entropy of the Linux kernel device /dev/urandom may be obtained using ioctl RNDGETENTCNT - that's what std::random_device::entropy() in GNU libstdc++ uses as of version 8.1

So under Linux ang g++ >= 8.1, I should be good... but I'm not:

#include <random>
#include <iostream>

void drain_entropy(std::random_device& rd, std::size_t count = 1)
{
    while (count --> 0) {
        volatile const int discard = rd();
        (void) discard;
    }
}

int main()
{
    std::random_device rd;
    std::cout << "Entropy: " << rd.entropy() << '\n'; // Entropy: 32
    drain_entropy(rd, 1'000'000);
    std::cout << "Entropy: " << rd.entropy() << '\n'; // Entropy: 32
}

Live demo on Coliru (which runs under Linux, right?)

What's happening?