SIGTERM is not captured by sigwait() - c++

Question

I'm writing a program that replys on SIGTERM/SIGINT to terminate. This is a multi-thread program. Basically I have a dedicated thread that is waiting for SIGTERM and SIGINT by calling sigwait and I will notify the main thread upon receving the signals in this thread.
I sent the SIGTERM signal to this program by executing command "killall nameofprocess". However, I found that it will work as expcted only in 10%. Most of times I have to run the "killall" command twice to terminate this program. I'm wondering why the sigwait doesn't capture the SIGTERM the first time. (Same case for SIGINT when I run kill -2)
Is it possible that the signal is received by some other threads?
static std::atomic<bool> shutdown_requested(false);
static std::mutex cv_mutex;
static std::condition_variable cv;
int main()
{
sigset_t sigset;
sigemptyset(&sigset);
sigaddset(&sigset, SIGINT);
sigaddset(&sigset, SIGTERM);
pthread_sigmask(SIG_BLOCK, &sigset, nullptr);
....do something including creating some threads.....
//a dedicated thread that waits for signals
auto signal_handler = [&sigset]() {
int signum = 0;
// wait until a signal is delivered:
sigwait(&sigset, &signum);
shutdown_requested.store(true);
// notify all waiting workers to check their predicate:
cv.notify_all();
return signum;
};
auto signal_handler_async = std::async(std::launch::async, signal_handler);
//wait for cv
while(shutdown_requested.load() == false)
{
std::unique_lock lock(cv_mutex);
cv.wait(lock, [](){return shutdown_requested.load();});
}
..........clean up and exit..............
return 0;
}