I'm seeing the same issue as this question, but I'm on SDK 13.1, which has a different peer_manager.c, so I can't directly use the fix for that question.
Like the other user, I'm getting PM_LINK_SECURED_PROCEDURE_ENCRYPTION after the devices use previously stored Bonding data and then using pm_conn_sec_status_get(), which gives p_conn_sec_status.bonded == 0.