lqi (the quality of the signal received) is usually calculated from a combination of the signal strength (rssi) and the number of errors. with rssi, -40 is better than -50, but sometimes RSSI is high because you’re actually receiving the interferer’s signal.
With Zigbee devices, it might be fixed just by moving a couple of meters in One Direction or another, just like Wi-Fi dead spots in the home.
I have a Wi-Fi booster and when it’s placed on the west wall of one room all the smartthings-controlled zigbee devices to the west of it fall off the network. If I just move it 90 degrees to the north wall, suddenly everything works again.
When I was in college, one of our exams was to imagine an empty house, fully networked, where everything worked great until a family moved into the house. We then had to identify everything that might be blocking signal. Most people got most things, but everyone, including me, missed a cast-iron frying pan that was sometimes put on two different shelves in the kitchen. On shelf A, everything worked fine. One shelf down and about half a meter to the right, and suddenly signal was blocked. It happens.
So given that you’re seeing a very low Quality of signal on one device, I would look for areas of local interference. If you can’t reproduce the quality issue with any of the other devices, it could just be a bad device. That also happens.