Any chance of resolving this without full reset?
I'm seeing something similar, 9.5.5 with magisk and Nova, upgraded from 9.5.4. I did full flash of 9.5.5 OTA (ie. all partitions not incremental) without wiping data.
I can only get Assistant to react when the screen is off or when the screen is "half on" (ie. black screen with clock). Other places it does not work, ex. lock screen, home screen, in the assistant app, in the Google app etc..
In 9.5.4 I could get the assistant to react from the home screen but never when using a BT headset.