Unwanted "Leave" command from coordinator

Hello, I developed for my customer, a ZigBee coordinator in last 2019, using nRF5 SDK for Thread and Zigbee v3.2.0.
Now my customer has this issue:
- End Node connected via router, if router deads, the coordinator rejoin it but after few time send "leave" command to end node
Here attached WireShark log file of ZigBee radio communication
Some details:
- end node has NW address E3DF
- router has NW address D01B
- at line 14631 and following, the router deads
- at line 14680 starting end node routing search
- at line 14747 and followings rejon procedure
- at line 14850 Coordinator sends LEAVE command to end node E3DF

What is happened? Why Coordinator sends "Leave" command?
How can we avoid this unwanted behaviour?

Many thanks for your help!