This is an issue that seems to have got more prevalent over time.
I primarily use my QC35 1 on my PC and phone. Turn sounds off on and you turn sound on the other and hey presto the sounds has transferred.
But now it seems to get stuck. Nothing will be playing on the PC but the bose app with not go back to the mobile because "windows sound" is playing. Other times you will be listening to something on the phone and bam got to connect to the PC to make the notification sound. Other times it goes the other way. No sound should be playing from the mobile but no bose thinks it is a priority now so nope, This takes priority.