XBEE S2C module looses its network

I have configured 16 end device to 1 coordinator network. The network was working well for 3 days continuously even if the coordinator is turned OFF and ON,end devices did not leave the network.
After 3 days of continuous testing, I turned off the coordinator for some duration of around 10mins, all 16 end devices did not come into network again. the system was left as it is, randomly the end devices one by one joined the network again after a duration of around 6-7hrs.
This problem has been observed for the first time.
I am not able to understand whether the issue was with end device or coordinator.
Kindly guide as i am getting same problem on site where the end device is leaving its network on its own and joins after some duration again on its own, without any kind of power ON/OFF of coordinator or end device.
Does anyone among the coordinator or end device tends to leave under any circumstances.
The module I am using is XBP24CZ7UIT-004-revD

This is by design and part of the Zigbee standard. Please see the Joining/rejoining section of the product manual.