I may have seen this as well. I had a MIDI track controlling external gear, thru track for the audio. I don’t remember the circumstances, because I assumed I had messed something up. I think I might have shut down or switched projects then back. And I had no audio. I could see the AB lights that the audio was coming in. Then after a while, when I was trying different things, the sound came back. Not sure what I did that made it work again.