This is normal.
Engbird compatible sensors are: IBS-TH1 mini, IBS-TH1, IBS-TH1 PLUS, IBS-TH2, IBS-TH2 PLUS, IBS-P01B, ITH-12S.
Due to the limitation of mobile phone permissions, Engbird cannot obtain Bluetooth information in the background/screen-off state.
It requires a sensor to be connected within Bluetooth range to get real-time readings. Therefore, only the Engbird app is running in the foreground and connected to the sensor, the alert notification will be triggered if the reading exceeds the alarm range.