It is a little complicated to describe this situation.
This test set up same like: "Thread Border Router using Linux + KW as Host Controlled Device with Serial TAP for DHCPv6-PD"
1. a REED "thr create" to start a new network, and then this REED becomes a LEADER and commissioner.
2. a "thread host controlled device" as a border router(BR), "thr join", and success to join this new network.
3. after few second, thr BR requesting become a router, and router Id assigned.
4. disconnecting REED(the LEADER) from the network, should be used "thr detach" or "reboot".
5. after few minutes, BR transfer its role to "Router/Leader".
6. REED "thr join" again, and success attached. immediately "Requesting to become Active Router".
but REED never gets router Id assigned, and show up "Requesting to become Active Router" over and over.
the REED try to join,
the attached file was captured from Wireshark.
an ED tries to join:
at this moment this network seems to dead.
7. power off or "reset" BR, after a moment,
REED show up "Node has taken the Leader role, (Local) Commissioner Started".
after that, everything smooth again
8. an ED "thr join" success, show up "Attached to network with PAN ID: xxxx".
9. BR "thr join", and success to attaching network. later "Device is Router, Router Id assigned".