Comment 8 for bug 1917099

So, I agree that you should be able to do a sub-zone delegation.

However, this works for me, so I'm curious what is causing your issue. I am running the master branch of Wallaby:

commit 37b3fa2ea454183e4a5c6a099eca5129959d10e1 (HEAD -> master, origin/master, origin/HEAD)

$ openstack zone create --email <email address hidden> parent.example.com.

+----------------+--------------------------------------+
| Field | Value |
+----------------+--------------------------------------+
| action | CREATE |
| attributes | |
| created_at | 2021-03-22T21:14:24.000000 |
| description | None |
| email | <email address hidden> |
| id | bcfa2e0e-e26a-4e83-a635-05fedada47ac |
| masters | |
| name | parent.example.com. |
| pool_id | 794ccc2c-d751-44fe-b57f-8894c9f5c842 |
| project_id | 270d45a47a104f95a7c669c694f2ffc6 |
| serial | 1616447664 |
| status | PENDING |
| transferred_at | None |
| ttl | 3600 |
| type | PRIMARY |
| updated_at | None |
| version | 1 |
+----------------+--------------------------------------+

$ openstack recordset create --record ns2.example.com. --type NS bcfa2e0e-e26a-4e83-a635-05fedada47ac child

+-------------+--------------------------------------+
| Field | Value |
+-------------+--------------------------------------+
| action | CREATE |
| created_at | 2021-03-22T21:15:22.000000 |
| description | None |
| id | cedd9862-d253-4b68-95a7-c43c61df313e |
| name | child.parent.example.com. |
| project_id | 270d45a47a104f95a7c669c694f2ffc6 |
| records | ns2.example.com. |
| status | PENDING |
| ttl | None |
| type | NS |
| updated_at | None |
| version | 1 |
| zone_id | bcfa2e0e-e26a-4e83-a635-05fedada47ac |
| zone_name | parent.example.com. |
+-------------+--------------------------------------+

; Zone dump of 'parent.example.com/IN'
;
parent.example.com. 3600 IN SOA ns1.devstack.org. me.here.com. 1616447721 3530 600 86400 3600
parent.example.com. 3600 IN NS ns1.devstack.org.
child.parent.example.com. 3600 IN NS ns2.example.com.

This looks like it works for me, do you see a difference? What version are you running?