For anyone in the US experiencing issues about their thermostat's Home Away Assist feature and had gone through the troubleshooting guide here: https://support.google.
Nest Learning Thermostat Display (3rd Generation)
Android BB Key2 OS 8.1.0 (also my wife has iPhone 10)
Issue started on 9 April. Previously also had a similar issue a few months ago with my wife's iPhone not reporting its location, but we were able to fix that. Note that if I set up the Nest app to be the one to use Home/Away Assist on my phone, I see the notification when I leave home or come back home...and see the events in the history. But it has no effect on turning the thermostat from Away to Home or vice versa. When instead settings up the Home app to use the phone for presence sensing for Home/Away Assist, the phones location doesn't seem to be recorded by the app at all. Note that when this function was working before 9 April, the Home app was the app that was set to control the Home/Away Assist on both my Android phone and my wife's iPhone.