I am also experiencing this. Google Home, whether via the Android app or a Google Home Mini, says all devices connected to Hubitat are offline when I try to send a command (on, off, etc). I've watched for log traffic on the Hubitat while trying this and don't see anything.
This is the first time I'm trying to control devices from Google Home via Hubitat. Previously I used Alexa. I can control other wifi-connected devices supported by Google Home, like GoSund switches, and discovery seems to work, I just can't control anything.
I've looked on my router for blocked traffic and see no indication that anything's wrong. It would help to know how Google Home is connecting to Hubitat - I assume through my account using the cloud connection.
...and, mysteriously, about 16 hours later, I am able to control Hubitat devices from the Google Home app, and (not too surprisingly) from my Google Home devices.
Apparently there's a substantial delay between adding the Hubitat (or changing it) and being able to communication with Google Home.
In short, the problem went away by itself after about 16 hours.