I recently made some changes to my app.yaml file (for a Node.js project) and decided to extend my health check pings to 4 minute intervals. According to the app.yaml documen
Known problem, see issue 36024384.
The value configured in the app.yaml
file is apparently multiplied by a value. More recently, including in your case, the multiplier appears to be 30.
So try to set it to 8 ;) And keep an eye on the issue (star it to get email updates) as when the fix will be deployed you'd probably want to update the configured value.