Wrong values in AVPlayerItemAccessLogEvent object

i'm using an AVPlayer object to read HLS streams. What I need to do is to extract metrics (startup time, number of stalls) from a streaming session.

Sometimes the playback is stalled. I know that because I added observer (AVPlayerItemPlaybackStalledNotification) on AVPlayerItem. But, when i check the numberOfStalls property in AVPlayerItemAccessLogEvent object, it always worth 0.

Thanks.