Background service BLE and MQTT

I have a app that needs to work all the time. It is very simple: Dataflow: From Bluetooth-App with MQTT Client in Phone-Cloud MQTT Broker- Cloud Backend-MQTT Broker-App with MQTT Client in Phone. All background services is allowed on ios Phone. I do not know when it started, but now suddenly the App stops randomly in background mode. The solution is built on that BLE scanning and App mqtt client is working. This "sleep" of the App make the system not working 100% and it is useless. Why is it stopping even if background is allowed? This must be fixed in iOS!!!!