Bonding. Why ltk changed at every reconnection?

Question

Hi,
I have done the bonding and saving context to flash. As an example I used ble_app_template. I cutted all redundant code and limited to single bond for simplicity.
In my code I save next: 
 - m_connection_table (I changed it a little bit)
 - m_peer_table 
 - m_bond_table (i.e. ble_gap_enc_key_t, LTK)
 
 At device startup I load m_peer_table to know what peer address I'm waiting to connect.
Advertice
Than after peer connect I load m_bond_table with presaved LTK.
At BLE_GAP_EVT_SEC_PARAMS_REQUEST peer central gives: 
 ble_evt->evt.gap_evt.params.sec_params_request.peer_params.kdist_periph.enc - 1
 ble_evt->evt.gap_evt.params.sec_params_request.peer_params.kdist_central.id - 0
 ble_evt->evt.gap_evt.params.sec_params_request.peer_params.kdist_periph.sign - 1
 ble_evt->evt.gap_evt.params.sec_params_request.peer_params.bond - 1
 
 My local setup is 
 m_sec_params.kdist_periph.enc = 1;
m_sec_params.kdist_periph.id = 0;
m_sec_params.kdist_periph.sign = 0; //Not supported yet
 
 At BLE_GAP_EVT_CONN_SEC_UPDATE 
Established Mode 0x01, Level 0x02 - so link is encrpted.Here I apply service context. 
 At BLE_GAP_EVT_AUTH_STATUS if bond successfull I save context to nonvolatile memory. But here the LTK is different whan I sent as reply to BLE_GAP_EVT_SEC_PARAMS_REQUEST . 
 After host disconnected I save all context similar to at BLE_GAP_EVT_AUTH_STATUS event. 
 As I understand LTK have to be constant for long time, so why it changes at each reconnection? It seem that I repeat bonding procedure each time at connection because I do not receive BLE_GAP_EVT_SEC_INFO_REQUEST at reconnect as stated here [https://devzone.nordicsemi.com/documentation/nrf51/4.4.0/html/group___b_o_n_d_e_d___m_a_s_t_e_r.html] or here [https://devzone.nordicsemi.com/documentation/nrf51/6.0.0/s110/html/a00824.html]

Valer_I · Accepted Answer

I tested on Win8.1 and it works, on Nexus7 doesn't. The test case was the next: 
 
 
 
 bonding 
 
 
 disconnection from host by switching off bluetooth or removing the BLE dongle. 
 
 
 On periperal wthite list become working and after switching on bluetooth on central, it connects and in one case rebonds with new LTK, in other case using previous bond. 
 
 
 When I receive BLE_GAP_EVT_DISCONNECTED the
ble_evt->evt.gap_evt.params.disconnected.reason is BLE_HCI_REMOTE_USER_TERMINATED_CONNECTION for Nexus7 and BLE_HCI_CONNECTION_TIMEOUT for Win8.1. Could this parameter be connected with described issue? 
 
 I think I found.
In Android code each time I connected the method called 
 device.createBond();
 
 Now it changed to and seems to work properly, but it not always define bond status correctly((( 
 if (device.getBondState() != BluetoothDevice.BOND_BONDED) {
 device.createBond();
}