I remember the entry in release note which is published at 2019-12-09. It should be related with this behavior.
>December 9, 2019 >App Engine is changing the URLs that you use to send requests to your apps. >You can now include a region ID to help Google route your requests more >efficiently and reliably. For example, an app can receive requests at >https://PROJECT_ID.REGION_ID.r.appspot.com. This new URL is optional for >existing apps, and will soon be required for all new apps. But now the entry seems to be vanished. Why? -- You received this message because you are subscribed to the Google Groups "Google App Engine" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/google-appengine/c0936434-97be-414d-8b56-1041dd0a82be%40googlegroups.com.
