Empty vip_subnet_id value causes octavia client exception

When creating a Kubernetes Cluster with API/etcd Load balancing via. Horizon and explicitly setting a fixed network during the creation process the operation fails with the following error.

Create_Failed: Resource CREATE failed: OctaviaClientException: resources.etcd_lb.resources.loadbalancer: Invalid input for field/attribute vip_subnet_id. Value: ''. Value should be UUID format (HTTP 400)

How to reproduce.

1. Container-Infra -> Cluster -> Create
2. Network -> Enable Load Balancer for Master Nodes
3. Network -> Uncheck "Create new Network"
4. Network -> Select already existent Network
5. Create

Cluster creation works for me in the following cases.

1. Let magnum automatically create a network
2. Set the fixed network and subnet inside the k8s template (does not work without subnet set)
3. Using the CLI

Openstack Version: Victoria
magnum --version 3.2.1