why does ZED could change its parent with Coordinator

Hi  all

          there are  several ZEDs which were  child nodes of  the  ZR1。

          when  ZR1  power off ， those child nodes  could  not  change parent  with coordinator。

          i  saw  those ZED send the orphan notification and  then  always sending the beacon request 。。。

          but  they could  not  join the network again， until   ZR1 power on again。

BR!