I, too, have about 20 Xiaomi sensors (door/window and motion) and have not had a single one drop off. I have a few other Zigbee devices (Iris buttons, Hue hub and bulbs), but nothing that repeats for these sensors.
Some of my sensors are a decent distance up the stairs, through numerous walls, and all check in hourly.
You may want to try healing your Zigbee network? Several ZigBee Devices stopped responding (Troubleshoot question)