Description:

• When central initiates bonding after connection established, the controller send failure event due not having the LTK for the connection immediately, and doesn't wait the host for sending it to the controller.
• Gap_Bond must be executed between the connection and the starting the bonding process to provide the connection handle the needed LTK.
• if there is no enough time , then providing the keys is impossible

Reproduce instruction:

flash host_test on peripheral device.

• connect with central (via HCI)
• initiate bonding from central - 
• peripheral will invoke this event "HCI_Command_Complete_LE_Long_Term_Key_Requested_Negative_Reply_Event"