I am facing same problem. What i have found is that when my app using getUserMedia in WKWebview is in background and i use different app using audio (for example audio/video player and play media) then after media is finished, i can hear again audio from my app BUT also my mic is working in background. What is different to getUserMedia in Safari is that when active mic in Safari is in background, i can see orange bar. In my app i can see only orange dot even if i get it working with the other app as mentioned before.