Possible Cause:

Keeping Engineering Mode enabled for an extended period can overload the UART bus. This may cause the LD2410 sensor to stop outputting valid data or produce incorrect readings.

Use Engineering Mode only temporarily for testing or fine tuning the sensor.

After completing tests, disable Engineering Mode to prevent UART congestion and ensure stable sensor operation.

Factory reset the sensor:

Install the HLKRadarTool app on your phone and connect to the radar via Bluetooth.
Tap the "Initialize" button (its name may be a little confusing) to reset the radar and restart the device.
If values still show "unknown", unplug the device for 3 seconds and plug it back in. Give Home Assistant a moment to update.

Note: If you notice that Engineering Mode turns off on its own, this is normal. In firmware v1.2 and newer, it is automatically disabled after 1 hour.