Runtime not exited after `std::async`

I build with the following options an autotest with emscripten 3.1.35:
`pthread PROXY_TO_PTHREAD emrun EXIT_RUNTIME ASYNCIFY`

Then I have a time-long method I want to execute.
If directly executed in the `main()`, the runtime exits correctly at the end of the test.
```C
int main(int argc, char** argv)
{
double result{0.};
for(auto i=0 ; i<100'000'000 ; ++i)
result += std::cos(i);

std::cout << "Result is " << result << "\n";
}
```

If doing the same with `std::async`, the runtime is never exited (then the test result cannot be known by the IDE)
```C
int main(int argc, char** argv)
{
auto result = std::async([]() -> double {
double result{0.};
for(auto i=0 ; i<100'000'000 ; ++i)
result += std::cos(i);
return result;
});
std::cout << "Result is " << result.get() << "\n";
}
```

I would expect that the program ends also in that case.
How can I make it exit ?