In past I try to solve this problem:
Now I have new informations about this problem.
Testing on hardware moduls ebyte e73-2g4m08s1c and fanstel bt-840 which contains NRF-52840.
this moduls have onboard or external antenna
Tested using SDK:
-simple coap client
-simple coap server
-dfu client + iot coap
Size of network:
2 - 8 nodes
verifyed through thread topology monitor
nodes are in one room, two furthest distance is 1-6 meters
Count of nodes which is unstable
count of unstable / size of network
sometimes [2 / 2]
sometimes [1 / 2]
sometimes [1 / 3]
sometimes [4 / 5]
sometimes [2-3 / 8]
OT DEVICE ROLES:
role number of role
Time to go UNSTABE
sometimes 1min, sometimes 5min, sometimes more...
Desctiption of behaviour:
-I create THREAD network using some example from SDK 
-nodes create THREAD mesh network 
-after time some nodes go from ROLE 2 or 3 or 4 to ROLE 1 
-sometimes one of nodes in ROLE 1 go to ROLE 4 and pbobably create separate THREAD mesh network and others join
-normally nodes which is unstable stay in ROLE 1 unless is device restarted
-is used DRV TIMER 2
-is used UART 2
-set Transmit Power to +8dBm
-thread CLI init is not call becouse need use these pins for my UART
- I use COAP protocol to comunicate with NCP
-NCP never have this problem
What do you mean by "the node becomes unstable"? Do you have any sniffer logs when this is happening you can share? I recommend you use a nRF52840 dongle together with our nRF Sniffer for 802.15.4 for Wireshark.
Unstable means that node go to role OT_DEVICE_ROLE_DETACHED and dont respond on mesages. When I try capture communication, sender node send message, but receiver node ignores all messages.
Now I use Thread Topology monitor for monitoring connections between nodes.
I notice some atributes that have node which described above problem
-node (role - OT_DEVICE_ROLE_ROUTER) comunicating and participating ind mesh network
-node become lost all conections with others nodes
-node deleted from TTM topology
-after cca 1 min node change role to OT_DEVICE_ROLE_DETACHED
What reasons allow node change role to OT_DEVICE_ROLE_DETACHED?
There could be many reasons for a node to go out of the network. The OT_DEVICE_ROLE_DETACHED only means that the node is not currently participating in a Thread network or partition.
Can you attach a sniffer trace and debug logs printed when this is happening?
First wireshark capture with 8 devices without rtt viewer
there is captured file from nrf Sniffer
used 8 devices:
NCP and device 6008 is started at the begining of capture. Others devices is still turned off.
after packet no. 108 start device 6002.
after packet no. 184 start device 6005
after packet no. 320 start device 6003
after packet no. 459 start device 6006
after packet no. 656 start device 6004
after packet no. 806 start device 6001
In this capture I monitor LED1 on all devices. LED1 signalize that:
-if device is in role 0 or 1 (OT_DEVICE_ROLE_DISABLED or OT_DEVICE_ROLE_DETACHED ) than not lights
-and when is in other role (2, 3, 4 - CHILD, ROUTER, LEADER) than lights
-between packets no. 459-561 LED1 stop lights and approximately after packet 653 start lights
-between packets no. 806-947 LED1 stop lights and approximately after packet 1024 start lights
-between packets no. 1042-1053 LED1 stop lights and approximately after packet 1095 start lights
-between packets no. 1126-1150 LED1 stop lights and approximately after packet 1172 start lights
-between packets no. 1192-1204 LED1 stop lights and approximately after packet 1239 start lights
-between packets no. 1320-1327 LED1 stop lights
-between packets no. 1095-1113 LED1 stop lights and approximately after packet 1154 start lights
-between packets no. 1172-1176 LED1 stop lights and approximately after packet 1208 start lights
-between packets no. 1239-1242 LED1 stop lights and approximately after packet 1327 start lights
-between packets no. 1095-1126 LED1 stop lights and approximately after packet 1157 start lights
-between packets no. 1172-1182 LED1 stop lights and approximately after packet 1212 start lights
-between packets no. 1239-1244 LED1 stop lights and approximately after packet 1327 start lights
between 1240-1320 my communication- my custom hello pacet from NCP to multicast, ncp gets only 3 responses(3/7)
Second wireshark capture with 3 problem devices (6002, 6006, 6003) with rtt viewer on device 6006
there is logfrom rtt viewer