Not sure what is going recently with people adding the sensors. Is everyone using catchall? I would avoid this method and persevere with adding them via the App directly as a Thing. The catchall method assigns a temporary Network ID which WILL change if the sensor loses contact with the hub temporarily or even if the hub is rebooted. Using the proper method it picks up the unique sensor ID. I have about 30 Xiaomi sensors ALL of which are rock solid (well bar one which is very far from the hub and drops off once in a while, quick few presses of the button and it is back though)
Also, if you have ANY other hard Wired Zigbee devices (sockets, bulbs etc.) then Unplug them from the mains before adding any of these sensors. Zigbee will route through the closes available gateway which in turn can cause issues, you want to initially pair them directly with the ST hub so take everything else out of the equation.
There is always obviously the case that ST have made changes to the Cloud which is impacting adding the sensors but we did have reassurance that as they now know what caused the major issues a few months back that they will try to prevent this from happening in the future.