I sort of have the same problem with ok google. Everytime I go to enable the "Always On" detection it switches it self off as soon as I leave the screen. If I manually do a voice search the mic works fine. So for me I think it's more of a software issue. The Mics works fine when recording video as well.
Did you by any chance select the Launch Google Assistant option for the Edge Sense and then changed it to something else? Because that turned of anywhere detection of the assistant and I had to go and enable it and train my voice again...
I haven't touched the edge sense settings so it is still using the original settings.
I've just tried to retrain voice which goes through ok, then go out of the settings and back in and the anywhere detection has turned back off!
Very strange