custom_http_statuses is not working

[yktakaha4]

Even if custom_http_statuses is defined, it is not reflected in the environment.

  custom_http_statuses {
    up = ["403"]
    down = [
      "200", "201", "202", "203", "204", "205", "206", "207", "208", "226",
      "300", "301", "302", "303", "304", "305", "306", "307", "308",
    ]
  }

Terraform used the selected providers to generate the following execution plan. Resource actions are indicated with the following symbols:
  + create

Terraform will perform the following actions:

  # uptimerobot_monitor.should_forbidden_http["xxx"] will be created
  + resource "uptimerobot_monitor" "xxx" {
      + friendly_name     = "xxx"
      + http_method       = "GET"
      + id                = (known after apply)
      + ignore_ssl_errors = false
      + interval          = 300
      + status            = (known after apply)
      + timeout           = 30
      + type              = "http"
      + url               = "xxx"

      + alert_contact {
          + id = "xxx"
        }

      + custom_http_statuses {
          + down = [
              + 200,
              + 201,
              + 202,
              + 203,
              + 204,
              + 205,
              + 206,
              + 207,
              + 208,
              + 226,
              + 300,
              + 301,
              + 302,
              + 303,
              + 304,
              + 305,
              + 306,
              + 307,
              + 308,
            ]
          + up   = [
              + 403,
            ]
        }
    }

Plan: 1 to add, 0 to change, 0 to destroy.

Do you want to perform these actions?
  Terraform will perform the actions described above.
  Only 'yes' will be accepted to approve.

  Enter a value: yes

Apply complete! Resources: 1 added, 0 changed, 0 destroyed.

expected

actual