Samsung ST Zigbee Motion & Door Sensors become OFFLINE - **unavailable**

My Samsung Smartthings Zigbee sensors are started failing as unavailable one by one in this week. I have tried:

  1. resetting and re-adding sensors
  2. updating settings in the Smartthings web management console.
  3. other options from this forum and YouTube.

A service request is being opened a few days ago and I had no response since then. Except the Samsung ST sensors, sensors from other vendors are working as normal.

Are these Samsung ST sensors reliable?