Bug 1695861

https://github.com/ManageIQ/manageiq/pull/18309

https://github.com/ManageIQ/manageiq/pull/18634 (for Hammer)

PR #18634 is now replaced by https://github.com/ManageIQ/manageiq/pull/18673

I don't see any authentication associated with conversion host .
Please check 

irb(main):017:0> host = Host.find(4)
=> #<ManageIQ::Providers::Redhat::InfraManager::Host id: 4, name: "env-rhv43-02.cfme2.lab.eng.rdu2.redhat.com", hostname: "env-rhv43-02.cfme2.lab.eng.rdu2.redhat.com", ipaddress: "10.8.197.93", vmm_vendor: "redhat", vmm_version: "7.6", vmm_product: "rhel", vmm_buildnumber: nil, created_on: "2019-06-30 20:12:25", updated_on: "2019-07-01 11:11:49", guid: "caf21053-c45c-4409-97a5-ba8bffe4d04f", ems_id: 53, user_assigned_os: nil, power_state: "on", smart: 1, settings: {}, last_perf_capture_on: nil, uid_ems: "6d6cccb2-9db0-416b-bdfb-7727ba3ac9b5", connection_state: "connected", ssh_permit_root_login: nil, ems_ref_obj: "--- \"/api/hosts/6d6cccb2-9db0-416b-bdfb-7727ba3ac9...", admin_disabled: nil, service_tag: nil, asset_tag: nil, ipmi_address: nil, mac_address: nil, type: "ManageIQ::Providers::Redhat::InfraManager::Host", failover: nil, ems_ref: "/api/hosts/6d6cccb2-9db0-416b-bdfb-7727ba3ac9b5", hyperthreading: nil, ems_cluster_id: 14, next_available_vnc_port: nil, hypervisor_hostname: nil, availability_zone_id: nil, maintenance: false, maintenance_reason: nil, physical_server_id: nil>

irb(main):018:0> host.authentications.first
=> nil

Your test is wrong. The class Host represents hypervisors. The class for conversion hosts is ConversionHost.
Here is an example:

irb> ConversionHost.first.authentications.first
=> #<AuthPrivateKey id: 6000000000016, name: "rhelh09.v2v.bos.redhat.com", authtype: "v2v", userid: "root", password: nil, resource_id: 6000000000006, resource_type: "ConversionHost", created_on: "2019-07-02 14:54:02", updated_on: "2019-07-02 14:57:20", last_valid_on: nil, last_invalid_on: nil, credentials_changed_on: nil, status: nil, status_details: nil, type: "AuthPrivateKey", auth_key: "v2:{FtdyXiu9zqpamjgjLj3Th7tLgvO9HeEHPekhyqCtRE4Nti...", fingerprint: nil, service_account: nil, challenge: nil, login: nil, public_key: nil, htpassd_users: [], ldap_id: [], ldap_email: [], ldap_name: [], ldap_preferred_user_name: [], ldap_bind_dn: nil, ldap_insecure: nil, ldap_url: nil, request_header_challenge_url: nil, request_header_login_url: nil, request_header_headers: [], request_header_preferred_username_headers: [], request_header_name_headers: [], request_header_email_headers: [], open_id_sub_claim: nil, open_id_user_info: nil, open_id_authorization_endpoint: nil, open_id_token_endpoint: nil, open_id_extra_scopes: [], open_id_extra_authorize_parameters: nil, certificate_authority: nil, google_hosted_domain: nil, github_organizations: [], rhsm_sku: nil, rhsm_pool_id: nil, rhsm_server: nil, manager_ref: nil, options: nil, evm_owner_id: nil, miq_group_id: 6000000000001, tenant_id: 6000000000001>

irb(main):001:0> ConversionHost.first.authentications.first
PostgreSQLAdapter#log_after_checkout, connection_pool: size: 5, connections: 1, in use: 1, waiting_in_queue: 0
=> #<AuthPrivateKey id: 41, name: nil, authtype: "v2v", userid: "root", password: nil, resource_id: 1, resource_type: "ConversionHost", created_on: "2019-07-26 20:14:58", updated_on: "2019-07-26 20:15:31", last_valid_on: nil, last_invalid_on: nil, credentials_changed_on: nil, status: nil, status_details: nil, type: "AuthPrivateKey", auth_key: "v2:{4krU5HF3iz7tBEOgTOK2fIEZEQnitZbRKdN1VLQuJozHVU...", fingerprint: nil, service_account: nil, challenge: nil, login: nil, public_key: nil, htpassd_users: [], ldap_id: [], ldap_email: [], ldap_name: [], ldap_preferred_user_name: [], ldap_bind_dn: nil, ldap_insecure: nil, ldap_url: nil, request_header_challenge_url: nil, request_header_login_url: nil, request_header_headers: [], request_header_preferred_username_headers: [], request_header_name_headers: [], request_header_email_headers: [], open_id_sub_claim: nil, open_id_user_info: nil, open_id_authorization_endpoint: nil, open_id_token_endpoint: nil, open_id_extra_scopes: [], open_id_extra_authorize_parameters: nil, certificate_authority: nil, google_hosted_domain: nil, github_organizations: [], rhsm_sku: nil, rhsm_pool_id: nil, rhsm_server: nil, manager_ref: nil, options: nil, evm_owner_id: nil, miq_group_id: 1, tenant_id: 1, become_username: nil, become_password: nil, auth_key_password: nil>

irb(main):002:0> ConversionHost.second.authentications.first
=> #<AuthPrivateKey id: 42, name: nil, authtype: "v2v", userid: "root", password: nil, resource_id: 2, resource_type: "ConversionHost", created_on: "2019-07-26 20:15:40", updated_on: "2019-07-26 20:16:10", last_valid_on: nil, last_invalid_on: nil, credentials_changed_on: nil, status: nil, status_details: nil, type: "AuthPrivateKey", auth_key: "v2:{4krU5HF3iz7tBEOgTOK2fIEZEQnitZbRKdN1VLQuJozHVU...", fingerprint: nil, service_account: nil, challenge: nil, login: nil, public_key: nil, htpassd_users: [], ldap_id: [], ldap_email: [], ldap_name: [], ldap_preferred_user_name: [], ldap_bind_dn: nil, ldap_insecure: nil, ldap_url: nil, request_header_challenge_url: nil, request_header_login_url: nil, request_header_headers: [], request_header_preferred_username_headers: [], request_header_name_headers: [], request_header_email_headers: [], open_id_sub_claim: nil, open_id_user_info: nil, open_id_authorization_endpoint: nil, open_id_token_endpoint: nil, open_id_extra_scopes: [], open_id_extra_authorize_parameters: nil, certificate_authority: nil, google_hosted_domain: nil, github_organizations: [], rhsm_sku: nil, rhsm_pool_id: nil, rhsm_server: nil, manager_ref: nil, options: nil, evm_owner_id: nil, miq_group_id: 1, tenant_id: 1, become_username: nil, become_password: nil, auth_key_password: nil>