AudioUnit speech stop

Question

I am using a speech synthesizer using the Audio Unit, based on Core Audio examples . Everything works as expected, except that StopSpeech and StopSpeechAt do nothing.

Here are ways to talk and stop:

void
Synthesizer::speak( const string &text )
{
    mIsSpeaking = true;
    mLastText = text;

    NSString *ns_text = [NSString stringWithCString:text.c_str()
                                           encoding:[NSString defaultCStringEncoding]];
    CFStringRef cf_text = (__bridge CFStringRef)ns_text;

    CheckError( SpeakCFString( mAuChannel, cf_text, NULL ), "SpeakCFString failed" );
}

void
Synthesizer::stop()
{
    if ( isSpeaking() ) {
        mStopRequested = true;

        CheckError( StopSpeech( mAuChannel ), "StopSpeech failed" );
    }

    else {
        mIsSpeaking = false;
    }
}

I checked that StopSpeech is called on the same thread as SpeakCFString. Oddly enough, when I try to enter the StopSpeech call using the debugger, it skips it. Really strange, a voice message callback starts when StopSpeech is called, but speech continues independently.

Any idea what might happen, or what can I do for debugging? Is there a workaround I could use to temporarily disable the Audio Unit node?

This is on MacOS 10.12.5.