I'm seeing this on stable/train environments using RDO packages during scale down tasks [0]. The tripleo-ipa project uses become: true to cleanup hosts in FreeIPA when a node is deleted. The keytab we use to authenticate to FreeIPA requires root [1].

(undercloud) [stack@undercloud ~]$ openstack server list
+--------------------------------------+-------------------------+--------+------------------------+----------------+-----------+
| ID                                   | Name                    | Status | Networks               | Image          | Flavor    |
+--------------------------------------+-------------------------+--------+------------------------+----------------+-----------+
| 4ea9059b-f91e-4b16-a4af-70d96acd372e | overcloud-controller-0  | ACTIVE | ctlplane=192.168.24.18 | overcloud-full | baremetal |
| 624942fd-9855-4bca-87e5-b90d63917fee | overcloud-novacompute-0 | ACTIVE | ctlplane=192.168.24.22 | overcloud-full | baremetal |
| 0c12414c-6c3e-4cbd-89a1-30eb19ac1aed | overcloud-novacompute-1 | ACTIVE | ctlplane=192.168.24.6  | overcloud-full | baremetal |
+--------------------------------------+-------------------------+--------+------------------------+----------------+-----------+
(undercloud) [stack@undercloud ~]$ openstack stack list
+--------------------------------------+------------+----------------------------------+-----------------+----------------------+--------------+
| ID                                   | Stack Name | Project                          | Stack Status    | Creation Time        | Updated Time |
+--------------------------------------+------------+----------------------------------+-----------------+----------------------+--------------+
| 6de764db-8b5f-45c2-910f-e1ed4cdbeedc | overcloud  | c144927514e949b3936762c25f3dff28 | CREATE_COMPLETE | 2020-08-11T22:02:53Z | None         |
+--------------------------------------+------------+----------------------------------+-----------------+----------------------+--------------+
(undercloud) [stack@undercloud ~]$ openstack overcloud node delete --stack overcloud --yes 0c12414c-6c3e-4cbd-89a1-30eb19ac1aed
Deleting the following nodes from stack overcloud:
- 0c12414c-6c3e-4cbd-89a1-30eb19ac1aed
Waiting for messages on queue 'tripleo' with no timeout.
Config downloaded at /var/lib/mistral/overcloud
Inventory generated at /var/lib/mistral/overcloud/tripleo-ansible-inventory.yaml
Running ansible playbook at /var/lib/mistral/overcloud/scale_playbook.yaml. See log file at /var/lib/mistral/overcloud/ansible.log for progress. ...


PLAY [Clear cached facts] ******************************************************

TASK [Gathering Facts] *********************************************************
Wednesday 12 August 2020  12:59:20 +0000 (0:00:00.168)       0:00:00.168 ****** 
[WARNING]: Failure using method (v2_runner_on_start) in callback plugin
(<ansible.plugins.callback.tripleo.CallbackModule object at 0x7f6b935523d0>):
'show_per_host_start'
ok: [overcloud-novacompute-1]


PLAY [Gather facts from undercloud] ********************************************
skipping: no hosts matched

PLAY [Gather facts from overcloud] *********************************************

TASK [Gathering Facts] *********************************************************
Wednesday 12 August 2020  12:59:27 +0000 (0:00:06.739)       0:00:06.907 ****** 
ok: [overcloud-novacompute-1]

PLAY [Load global variables] ***************************************************

TASK [include_vars] ************************************************************
Wednesday 12 August 2020  12:59:32 +0000 (0:00:04.762)       0:00:11.670 ****** 
ok: [overcloud-novacompute-1]

PLAY [Ensure /var/lib/config-data context] *************************************

TASK [Ensure /var/lib/config-data context] *************************************
Wednesday 12 August 2020  12:59:32 +0000 (0:00:00.192)       0:00:11.862 ****** 
changed: [overcloud-novacompute-1]


PLAY [Render all_nodes data as group_vars for overcloud] ***********************

TASK [Render all_nodes data as group_vars for overcloud] ***********************
Wednesday 12 August 2020  12:59:33 +0000 (0:00:01.195)       0:00:13.057 ****** 
ok: [overcloud-novacompute-1]

PLAY [Set all_nodes data as group_vars for overcloud] **************************

TASK [Set all_nodes data as group_vars for overcloud] **************************
Wednesday 12 August 2020  12:59:36 +0000 (0:00:02.802)       0:00:15.860 ****** 
ok: [overcloud-novacompute-1]

PLAY [Scaling] *****************************************************************

TASK [include_tasks] ***********************************************************
Wednesday 12 August 2020  12:59:36 +0000 (0:00:00.147)       0:00:16.008 ****** 
included: /var/lib/mistral/overcloud/scale_steps_tasks.yaml for overcloud-novacompute-1

TASK [tripleo_ipa_cleanup : delete hosts, subhosts and services from freeIPA] ***
Wednesday 12 August 2020  12:59:37 +0000 (0:00:00.371)       0:00:16.379 ****** 
fatal: [overcloud-novacompute-1]: FAILED! => {"changed": false, "module_stderr": "sudo: unable to open /run/sudo/ts/mistral: Permission denied\nsudo: a password is required\n", "mo
dule_stdout": "", "msg": "MODULE FAILURE\nSee stdout/stderr for the exact error", "rc": 1}

PLAY RECAP *********************************************************************
overcloud-novacompute-1    : ok=7    changed=1    unreachable=0    failed=1    skipped=0    rescued=0    ignored=0   

Wednesday 12 August 2020  12:59:37 +0000 (0:00:00.519)       0:00:16.899 ****** 
=============================================================================== 

Ansible failed, check log at /var/lib/mistral/overcloud/ansible.log.
Scale-down configuration failed.


[0] http://paste.openstack.org/show/796775/
[1] https://opendev.org/x/tripleo-ipa/src/branch/master/tripleo_ipa/roles/tripleo_ipa_cleanup/tasks/main.yml#L28