During some tests I came across some odd results. Put Debug.Log (Thread.CurrentThread.ManagedId); into the OAFR callback and see what happens.
Requesting an explanation from Jan or UT please 
Not sure what I’m seeing. New thread id every callback?
indeed as Gregzo said:
AppDomain.GetCurrentThreadId: This method is marked as deprecated because the .NET Thread object does not correspond to a single Windows thread, and as such there is no stable ID which can be returned by Windows for a given .NET thread.
void OnAudioFilterRead(float[ ] data, int channels)
{
int id = System.AppDomain.GetCurrentThreadId();
Debug.Log( id );
}