My English is not good so, please read previews conversation between the author and @trohn_javolta. It'll make better sense.
Anyway, on Android TV ( -Android 9), multi apps can't access single microphone at the same time.
So you need to stop Hotword Plugin before Google.app access a microphone by using Tasker.
I still don't know what is the best approach to restart Plugin because Google.app does not tell when it release a mic.