Resolution
The Casc configuration gets overridden by the Operations Center. In CloudBees CI on Modern Platform, the Operations Center itself configures the Resource URL for the whole cluster. In addition to configuring the Resource root URL for all the Controllers in the cluster, it also takes care of creating the Ingress rules that you need for the feature to work.
You should therefore define the resource URL directly from the Operations Center.
To do so go to <operations_center_url>/masterProvisioning/ and set the resource url in the Resource host field.
Alternatively, if you want to set the Resource URL for a single controller or not use the same pattern across all the cluster you can set the -DMASTER_RESOURCE_URL System Property per controller.