I heard one of the google dev's for android tv was tired of the lack of formal support. Apparently he gave this entire speech at the cafeteria with a mic drop and everything.

on my google tv, pressing assistant button while keyboard is open activates voice typing

Go to keyboard settings and change the default keyboard. Possible that another keyboard got selected.

OP did you deny access to the microphone? If not, check the permissions on the Gboard app to see that it has all the applicable permissions enabled.

If not try to delete cache and data of your keyboard in settings/apps

This happened to me and I went into the Android TV (shield tv) and changed the keyboard. Turning leanback keyboard off.

Have you changed your remote from Bluetooth to IR by any chance? From what I recall the voice assistant requires BT in order to operate.

Try to reconnect your remote through the Bluetooth settings.

Call Google

I use an android TV remote on my phone to use the microphone