UnmixedAudioBuffers always null

Goal: I need to be able to get the participant/speaker specific audio media from the call.
I found that setting ReceiveUnmixedMeetingAudio to true will do the trick, but I am receiving null from it.
My other option is to retain getting MixedAudio and just query the media stream somewhere from https://graph.microsoft.com/v1.0/communications/callRecords/87b8106e-52df-4ef9-be7e-9616f0079007?$expand=sessions($expand=segments)
But I can't find any example on how to do it.

Any help will be appreciated. Thanks!

Logs:
OnAudioMediaReceived: AudioMediaReceivedEventArgs: {"Buffer":{"Data":{"value":1983124767808},"Length":640,"AudioFormat":"Pcm16K","ActiveSpeakers":[],"IsSilence":true,"Timestamp":38104121949150000,"UnmixedAudioBuffers":null}}

AudioSocketSettings:
var mediaSession = this.Client.CreateMediaSession(
new AudioSocketSettings
{
StreamDirections = StreamDirection.Recvonly,
SupportedAudioFormat = AudioFormat.Pcm16K,
ReceiveUnmixedMeetingAudio = true,
},
videoSocketSettings,
vbssSocketSettings,
mediaSessionId: mediaSessionId);

Start at the CreateMediaSession call using AudioSocketSettings and ReceiveUnmixedMeetingAudio, then inspect the OnAudioMediaReceived log where UnmixedAudioBuffers is null. Compare that behavior with the Microsoft Graph callRecords sessions/segments endpoint mentioned in the report; done means establishing why participant-specific audio is null or documenting the supported retrieval path.