ZED rejoin issue about ZC send leave Cmd

Hi  all

today  i found a ZED could not  rejoin the network although the LQI is good。

SETUP ：

ZED ===link === ZC

sometimes  the ZED  disconnect the network and scan try to rejoin.

Rejoin backoff ： 30minute

Rejoin scan ：3 minute

Child Aging mode ： POLL RATE

i found something strange  in the log ，

 ZED sending  one rejoin request， the ZC sends a Leave CMD for  it ，

 both of  them doing  this again and again   until   ZED go into the BACKOFF......

And  as i printf out the association list of ZC ，i found  this ZED isn't  in it ！！！！！

so  i turn off  the power of the ZED ，and  restart it again，  it send a orphan notification  and then ZC send a coordinator realignment 

and  then it sends device announce  and join the network！！！

My  question：

Q1：

       Why  does the ZC  sends  the Leave CMD while  ZED send rejoin request？

       when does ZC would sends  this CMD ?

Q2 :

       Why does ZED short address isn't  in  the association list of  the ZC ?

       what and when would caused  the ZC delete some node  of  the association list ？

Q3：

       Why  does  ZED could rejoin success to the network  after power off /power on without ZC turn on the permit Join， although there is not record on the ZC’s association list ？

BR!