stackit_ske_cluster: Provider returns unknown value for extensions.dns.zones after apply when dns.enabled=false

[kdambiec]

Description

After terraform apply, the provider returns an invalid result object for stackit_ske_cluster.extensions.dns.zones. Terraform reports that this attribute remains unknown even though all values must be known post-apply. Terraform saves other known values but flags this as a provider bug.
Error excerpt:
Error: Provider returned invalid result object after apply

After the apply operation, the provider still indicated an unknown value for stackit_ske_cluster.example.extensions.dns.zones. All values must be known after apply, so this is
always a bug in the provider and should be reported in the provider's own repository. Terraform will still save the other known object values in the state.

Steps to reproduce

terraform {
  required_version = ">= 1.6.0"
  required_providers {
    stackit = {
      source  = "stackitcloud/stackit"
      version = ">= 0.73.0"
    }
  }
}

provider "stackit" {
  # authentication config if needed
}

variable "project_id" {
  type = string
}

locals {
  example_ske_cluster_name = "tf-example-ske-cluster"
}

resource "stackit_ske_cluster" "example" {
  project_id             = var.project_id
  name                   = local.example_ske_cluster_name
  kubernetes_version_min = "1.33.6"

  node_pools = [
    {
      name               = "np-example"
      machine_type       = "g1a.2d"
      os_version         = "4230.2.3"
      minimum            = "2"
      maximum            = "3"
      availability_zones = ["eu01-2"]
    }
  ]

  maintenance = {
    enable_kubernetes_version_updates    = true
    enable_machine_image_version_updates = true
    start                                = "01:00:00Z"
    end                                  = "02:00:00Z"
  }

  extensions = {
    dns = {
      enabled = false
    }
    observability = {
      enabled = false
    }
    acl = {
      enabled = true
      allowed_cidrs = [
        "0.0.0.0/0"
      ]
    }
  }
}

resource "stackit_ske_kubeconfig" "example" {
  project_id     = var.project_id
  cluster_name   = stackit_ske_cluster.example.name
  refresh        = true
  expiration     = 7200 # 2 hours
  refresh_before = 3600 # 1 hour
}

1. Run terraform init
2. Run terraform plan
3. Run terraform apply
4. Observe the error reported after apply.

Actual behavior

Terraform reports:
│ Error: Provider returned invalid result object after apply
│
│ After the apply operation, the provider still indicated an unknown value for stackit_ske_cluster.example.extensions.dns.zones. All values must be known after apply, so this is
│ always a bug in the provider and should be reported in the provider's own repository. Terraform will still save the other known object values in the state.
╵

The cluster and other attributes appear to be created/saved, but the provider leaves extensions.dns.zones as an unknown value in the returned object/state even when extensions.dns.enabled = false.

Expected behavior

All attributes of stackit_ske_cluster, including extensions.dns.zones, should be known and finalized after apply. If dns.enabled = false, the provider should either:

Omit zones (if optional) or
Return a deterministic, known value (e.g., []), rather than an unknown placeholder.

Environment

OS: Windows (amd64)
Terraform version: v1.14.1
Providers:

registry.terraform.io/stackitcloud/stackit v0.73.0

Additional information

The issue occurs even when DNS extension is disabled (dns.enabled = false), which suggests the provider still tries to read or set extensions.dns.zones as unknown.
The behavior persists across repeated applies.
If needed, I can attach the Terraform state snippet for the affected resource and provider debug logs (TF_LOG=DEBUG).