Google Assistant behaves strangely to say the least. First you have to make sure it runs as a system app.
Follow this guide if you are running Magisk, it worked for me. If "Google" doesn't appear in the list after you run systemize, then it's probably running as a system service so there's no need to proceed with this guide and you can just close the window.
How to Fix “OK Google”/Access with Voice match option Greyed out
If it does run as a system service, you can always try to uninstall, register for the beta version, wait for a few minutes to get beta access and reinstall. See if that improves something. You can also delete all assistant + voice related activity on your google account.
Clicking here should get you to the proper page with the appropriate filters selected.
Just click on the dots next to Search and select "Delete results" to delete every recording for the Assistant.
As an example of Assistant weirdness, on the latest stable eu MIUI on my Rednote 7, Google Assistant can be "enabled", says there's already a voice model and after I click ok, it switches off again, then it won't work at all. I have to uninstall it, reinstall it and never go back to that option for it to work. I can't retrain or delete since it can't be enabled in options.
