The network resource provisioning playbook for 4.15 dualstack UPI contains a task for adding an IPv6 subnet to the existing external router [1].
This task fails with:
- ansible-2.9.27-1.el8ae.noarch & ansible-collections-openstack-1.8.0-2.20220513065417.5bb8312.el8ost.noarch in OSP 16 env (RHEL 8.5) or
- openstack-ansible-core-2.14.2-4.1.el9ost.x86_64 & ansible-collections-openstack-1.9.1-17.1.20230621074746.0e9a6f2.el9ost.noarch in OSP 17 env (RHEL 9.2)

4.15.0-0.nightly-2024-01-22-160236

Always

1. Set the os_subnet6 in the inventory file for setting dualstack
2. Run the 4.15 network.yaml playbook

Playbook fails:
TASK [Add IPv6 subnet to the external router] ********************************** fatal: [localhost]: FAILED! => {"changed": false, "extra_data": {"data": null, "details": "Invalid input for external_gateway_info. Reason: Validation of dictionary's keys failed. Expected keys: {'network_id'} Provided keys: {'external_fixed_ips'}.", "response": "{\"NeutronError\": {\"type\": \"HTTPBadRequest\", \"message\": \"Invalid input for external_gateway_info. Reason: Validation of dictionary's keys failed. Expected keys: {'network_id'} Provided keys: {'external_fixed_ips'}.\", \"detail\": \"\"}}"}, "msg": "Error updating router 8352c9c0-dc39-46ed-94ed-c038f6987cad: Client Error for url: https://10.46.43.81:13696/v2.0/routers/8352c9c0-dc39-46ed-94ed-c038f6987cad, Invalid input for external_gateway_info. Reason: Validation of dictionary's keys failed. Expected keys: {'network_id'} Provided keys: {'external_fixed_ips'}."}

Successful playbook execution

The router can be created in two different tasks, the playbook [2] worked for me.