My Quiet Comfort 35 II headphones only get audio from the window I'm taking a call from if I'm on a call. For example, if I'm in Discord and I'm also playing a game on my computer I can only hear the audio from Discord. My headphones have just started doing this yesterday, and have never done it in the 3 months I've owned them. I've tried restarting my PC and headphones, yet this has done nothing.
This community is for portable speakers - QC35s are normally discussed in 'around an on-ear headphones'.
As far as your question - from a troubleshooting perspective, ask yourself 'what has changed ' since it stopped working. Change in behavior implies something has caused it.
Though to be quite frank, I am personally surprised it has worked at all on both simultaneously (which could be the change 🙂 ). The QC35 bluetooth (BT), works with one of two BT standard profiles - A2DS for stereo streaming and HFP (hands free profile) for phone calls and voice over IP (VOIP) applications. The way Bose has implemented BT for the QC35ii as I understand it is that these two are mutually exclusive - that is, if a call comes through or you start a VOIP application, any music / sound playing in the QC35ii (from A2DS) will be temporarily stopped when it switches to HFP and the music will restart when HFP is done.