iOS11 AUGraphInitialize Err-code 560557684 - background

Question

With the application in the background on iOS 11,
Execution of AUGraphInitialize () fails,
Error code 560557684 is returned and audio can not be played.
AUGraphInitialize () was successful on iOS 10 in the same situation.
Is this a bug in iOS 11?
Is there planning to fix apple in this case?
If it is not a bug,
With iOS 11 the application is in the background,
What kind of implementation should be done to make AUGraphInitialize () succeed?
Please give me hints!
Describe the source of this error case below.
AVAudioSession *audioSession = [AVAudioSession sharedInstance];
if (audioSession == nil) {
LOGE("AVAudioSession : sharedInstance() error.");
}
NSError *audioSessionError = nil;
BOOL ret = [audioSession setActive:NO error:&audioSessionError];
if (ret == false) {
LOGE("AVAudioSession : setActive() error. code = %d", (int)audioSessionError.code);
}
NSTimeInterval bufferDuration = 2048 / 44100;
ret = [audioSession setPreferredIOBufferDuration:bufferDuration error:&audioSessionError];
if (ret == false) {
LOGE("AVAudioSession : setPreferredIOBufferDuration() error. code = %d", (int)audioSessionError.code);
}
ret = [audioSession setCategory:AVAudioSessionCategoryPlayback
error:&audioSessionError];
if (ret == false) {
LOGE("AVAudioSession : setCategory() error. code = %d", (int)audioSessionError.code);
}
[audioSession setActive:YES error:&audioSessionError];
OSStatus err = NewAUGraph(&mGraph);
if ( err != noErr ) {
LOGE("AUGraph : NewAUGraph() error. code = %d", (int)err);
}
err = AUGraphOpen(mGraph);
if ( err != noErr ) {
LOGE("AUGraph : AUGraphOpen() error. code = %d", (int)err);
}
AudioComponentDescription output_desc;
output_desc.componentType = kAudioUnitType_Output;
output_desc.componentSubType = kAudioUnitSubType_RemoteIO;
output_desc.componentManufacturer = kAudioUnitManufacturer_Apple;
output_desc.componentFlags = 0;
output_desc.componentFlagsMask = 0;
AUNode outputNode;
err = AUGraphAddNode(mGraph, &output_desc, &outputNode);
if ( err != noErr ) {
LOGE("AUGraph : AUGraphAddNode() error. code = %d", (int)err);
}
err = AUGraphNodeInfo(mGraph, outputNode, NULL, &mOutUnit);
if ( err != noErr ) {
LOGE("AUGraph : AUGraphNodeInfo() error. code = %d", (int)err);
}
AURenderCallbackStruct callback;
callback.inputProc = _renderCallback;
callback.inputProcRefCon = (__bridge void*)self;
err = AUGraphSetNodeInputCallback(mGraph,
outputNode,
0,
&callback);
if ( err != noErr ) {
LOGE("AUGraph : AUGraphSetNodeInputCallback() error. code = %d", (int)err);
}
AudioStreamBasicDescription outputFormat;
UInt32 size = sizeof(AudioStreamBasicDescription);
err = AudioUnitGetProperty( mOutUnit, kAudioUnitProperty_StreamFormat, kAudioUnitScope_Output, 0, &outputFormat, &size );
if ( err != noErr ) {
LOGE("AUGraph : AudioUnitGetProperty() error. code = %d", (int)err);
return nil;
}
outputFormat.mSampleRate = 44100.0f;
err = AudioUnitSetProperty(mOutUnit, kAudioUnitProperty_StreamFormat, kAudioUnitScope_Input, 0, &outputFormat, size);
if ( err != noErr ) {
LOGE("AUGraph : AudioUnitSetProperty() error. code = %d", (int)err);
}
err = AUGraphConnectNodeInput(mGraph,
outputNode, 0,
outputNode, 1
);
if ( err != noErr ) {
LOGE("AUGraph : AUGraphConnectNodeInput() error. code = %d", (int)err);
}
err = AUGraphInitialize(mGraph);
if ( err != noErr ) {
LOGE("AUGraph : AUGraphInitialize() error. code = %d", (int)err);
}