iOS12 portraitEffectsMatte is still nil with pixel format output?

Hey guys, I just start to use the API AVPortraitEffectsMatte. I often work around the AVDepthData. But I'm confused with the return value of portrait matte.

I want to capture the photo also get the portrait effect matte in the same time. When I use the fileDataRepresentationWithCustomizer, the matte works fine. I can get matte from photos just toke, and the callback photo.portraitEffectsMatte ISN'T nil. But when I want to capture the photo with pixelformat, and also set the setPortraitEffectsMatteDeliveryEnabled correctly, the photo.portraitEffectsMatte in callback is always nil. Why?

I tested it on my iPhone X with iOS 12. Here is the code I use.

First I enable the depth and portrait matte in AVCapturePhotoOutput.

if (photoOutput.isDepthDataDeliverySupported)
{
     [photoOutput setDepthDataDeliveryEnabled:YES];
}

if (@available(iOS 12.0, *))
{
     if (photoOutput.isPortraitEffectsMatteDeliverySupported)
     {
          [photoOutput setPortraitEffectsMatteDeliveryEnabled:YES];
     }
}

And I wrote this when I press the capture button.

AVCapturePhotoSettings * photoSettings = [AVCapturePhotoSettings photoSettingsWithFormat:[NSDictionary dictionaryWithObjectsAndKeys:[NSNumber numberWithInt:kCVPixelFormatType_32BGRA], kCVPixelBufferPixelFormatTypeKey, nil]];
if (photoOutput.isDepthDataDeliverySupported)
{
     [photoSettings setDepthDataDeliveryEnabled:YES];
     photoSettings.embedsDepthDataInPhoto = NO;
 }
if (@available(iOS 12.0, *))
 {
     if (photoOutput.isPortraitEffectsMatteDeliverySupported)
      {
          [photoSettings setPortraitEffectsMatteDeliveryEnabled:YES];
          photoSettings.embedsPortraitEffectsMatteInPhoto = NO;
     }
 }

[photoOutput capturePhotoWithSettings:photoSettings delegate:self];

Finally get this callback.

- (void)captureOutput:(AVCapturePhotoOutput *)output didFinishProcessingPhoto:(AVCapturePhoto *)photo error:(nullable NSError *)error
{
     if (@available(iOS 12.0, *))
     {
          NSLog(@"%@", photo.portraitEffectsMatte ? @"Matte": @"nil");
     }
}

But it's still nil.

How can I get the correct portrait matte with the pixelformat output?

Thank you.