The other day this problem started. Random Zig-Bee devices would go offline and nothing would bring them back until I did the following. I opened IDE went to my devices then the device. I then clicked on the device and choose edit. I picked a different device type similar to the one it uses. Clicked save and the device came back online. I then reverted back to original. Example SmartSense Motion Sensor I changed to SmartSense Motion.
This problem includes mostly SmartThings devices, but sometimes Lightify and IRIS plugs.