Crash when assigning to std::future under MSVC's Debug mode

The following code crashes in random intervals when built with MSVC under Debug mode, unlike the Release.

#include <future>

using namespace std;

int main() {
    auto l = [](){};
    auto f = async(launch::async, l);

    for (int i = 0; i < 1000000; ++i)
        f = async(launch::async, l);
}

The console output says:

f:\dd\vctools\crt\crtw32\stdcpp\thr\mutex.c(51): mutex destroyed while busy

The full call stack is: https://pastebin.com/0g2ZF5C1

Now obviously it's just a stress test, but am I doing something utterly stupid? It seems to me it's fine to reassign a new task to an existing future, as it's said that operator=:

Releases any shared state and move-assigns the contents of other to *this

(Due to http://en.cppreference.com/w/cpp/thread/future/operator%3D).

Is it a bug in MSVC's runtime?

Remarkably, the program stops crashing if I manually call wait() before the assignment, thus making the loop into:

for (int i = 0; i < 1000000; ++i) {
    f.wait();
    f = async(launch::async, l);
}

Isn't operator= itself supposed to call wait?

Background:

_MSC_VER equals 1911

Code was built with help of:

Microsoft Visual Studio Community 2017 Preview(2)
Version 15.4.0 Preview 2.0

Just opened a brand new C++ project.