Please provide a screenshot of your blocks as well as the project (aia file)
Taifun
you are trying to do something (in this case send a notification) regularly every 30 minutes
this is a job for an alarmmanager
search the community for a free basic alarmmanager or use my paid alarmmanager with advanced features App Inventor Extensions: Alarm Manager | Pura Vida Apps
Taifun
Earlier i built a mqtt app, which was stooping after sometime. Because of that I am sending notification message every 30 minutes with timestamp to see after how long the app stops.
Yes, unrestricted battery background usage is enabled
This should help a bit. It is a Motorola device-specific behaviour.
Keep the application open in recent menu and check if you have an option to "Lock app".