Based on the description you have provided, my best answer is that if your Outh Client ID and API key are still available in Cloud Dashboard(which you can check here: https://console.cloud.google.com/apis/api/gmail.googleapis.com/credentials?authuser=2&inv=1&invt=AboUDA&project=inboxiq-432403), I assume you may have gone over the allotted API credits.

It is also standard for OAuth Playground to delete access tokens after 3589 seconds, so that could be it.

Apologies I couldn't be more helpful.