Took me an embarrassing amount of time to figure this out.
There are two ways to connect Gmail in your automations. They look exactly the same. One randomly stops working every few months.
The quick sign-in method is what Google considers "testing mode." Your token expires. Your client's emails stop. You get a panicked message at the worst possible time.
Do the Google Cloud setup instead. Your own Client ID. Your own Refresh Token. It never expires.
20 minutes now saves you a very awkward client call later.