Is FAudioThread::RunCommandOnAudioThread executing synchronously?

Development Audio
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I’ve just updated a project to Unreal 4.16. This version forces me to call ome methods on the AudioThread and some others on the GameThread.
To do that I used

FAudioThread::RunCommandOnAudioThread

I need to use on the GameThread the result of the command executed on the AudioThread. To do that I imagined, maybe wrongly, that it was necessary to use some synchronization constructs in order to have the GameThread waiting the command executed on the AudioThread.

I tried to log something:

UE_LOG(MyLog, Log, TEXT("One"));
FAudioThread::RunCommandOnAudioThread([param1, param2]() {
    // my audio operations...
    UE_LOG(MyLog, Log, TEXT("Two"));
});
UE_LOG(MyLog, Log, TEXT("Three"));

The result is sometimes I have “One Two Three”, sometimes “One Three Two”. So I think the GameThread is not waiting for the AudioThread’s command.
The problem is that, if I use some synchronization constructs (a Semaphore for example), the GameThread become stuck. I see “One Two” and nothing else…

So, what’s the real behavior of RunCommandOnAudioThread?