Report it to Google.

The "Google" app is where Assistant lives, so the fact that restoring that app to the factory-installed version fixes your problem suggests that the current Google app has a bug and does not work properly with either the S9, your version of the S9 (all Samsung phones come in many versions), or maybe your individual S9 (i.e. is incompatible with some particular feature of your phone's apps/data/settings). And the fact that it does work again when you revert the version, then stops when you re-update, indicates that it's a problem with the app version rather than you having inadvertently changed some important setting. So that's my guess, but as we don't know how widespread this is we don't know whether Google are aware, hence my suggestion that you report it to them.

Meanwhile turn off automatic updates in the Play Store so that it doesn't just reinstall the broken version on your phone and use the old one for a bit.