I have an Android phone, but I am wondering if you are seeing something similar to me.
Do you have/use the MyPhonak app on your phone?
On mine, whenever the phone is activated (notifications, apps, or even unlocking the screen), the app switches my program from whatever I have it set to, to "Bluetooth Streaming +Mic.
There is an “Ambient Balance” button at the bottom that seems to adjust microphone volume (or maybe bias between mics and streamed sounds)
Moving the slider to the left will increase the volume coming from the mics. When it’s all the way to the right I get a symptom just like what you describe