I have the Quiet Comfort 35 II and I'm connected to my Macbook via the cable, attending a Zoom video conference. But only the side which is connected to the cable has sound. Microphone works fine, and when listening to other things on the computer, like Spotify, I have sound on both sides. What could be the issue? I've looked through the settings in Zoom but can't find anything.
First thing to check is another compatible cable just to rule out the possibility. Now you are talking about two different BT 'profiles'. A2DS which is stereo streaming is designed to use both speakers - left and right channels. But the voice over IP applications using HFP (Hands Free Protocol) such as Zoom, Whatsapp, Skype, etc. use the same approach as classic 'phones' - Mono. Now Mono can be produced in two ways - same audio signal across both channels (where everything seems to come from the center) or just on say the Left channel. I can't speak for all cars, but when I talk to people using HFP in my Toyota Prius, the sound comes only from the front left speaker on the dash.
So while I use a windows laptop and not a Mac, I am 'guessing' that the Mac for the HFP profile is sending out the mono audio only out of the left speaker. So where your journey takes you is to see if you can send an HFP signal across both channels. If it can be changed, it should be either associated with BT or the Audio settings.
I can tell you it does not likely have anything to do with the QC35 since I hear my phone conversations on both speakers/channels.