XBEE3 routers lost connections in time

We setup a ZB network with 16 Xbee3 routers connected to the coordinator. The router Xbee3’s are working as Transparent mode, The coordinator is running as API mode.
They are working fine for few weeks, then 4 clients drop the connections, one week later, another 4 units lost connections, then all of them lost connections to the coordinator. The ND command verified all of then didn’t join the coordinator.
BUT, after send NR=1 command to coordinator, all clients are back!

Any idea what could cause this kind issue? How do we debug this?
Also is there any way that we can retrieve ZB network layer logs?

No you can’t view the stack logs. But you should do more than a Node Discovery to determine a node is no longer on the network. Try sending a remote AT command to that node and see what it does.