GCF - evnironment variables are not changing when deployed from terraform

Question

I have a Terraform code that is deploying Google Cloud Function. When I modify environment_variables by changing the variable name e.g.: DOMAIN to APP_DOMAIN, the code in GCF is being updated, version is being increased, but the variables don't. I have to destroy the function and recreate it to make new env variable work.

resource "google_cloudfunctions_function" "pubsub_triggered_function" {
  region  = var.region
  name    = var.pubsub_triggered_function_name
  runtime = "python39"
  available_memory_mb   = 128
  source_archive_bucket = google_storage_bucket.code_archive_bucket.name
  source_archive_object = google_storage_bucket_object.archive.name
  entry_point           = var.function_entry_point
  service_account_email = "${var.service_account_email}"
  event_trigger {
    event_type = "providers/cloud.pubsub/eventTypes/topic.publish"
    resource = var.pubsub_topic
    failure_policy {
      retry = true
    }
  }

  environment_variables = {
    DOMAIN = var.app_domain
    ENVIRONMENT = var.environment
    PROJECT_ID = var.project
  }
}

When I change this manually, the variables do change and new version of the cloud function has new variable. It's seems to be a Terraform bug. (Terraform v1.1.5)

--- EDIT #1 ---

Adding the first part of the code - zip archive might be relevant.

locals {
  timestamp = formatdate("YYYY-MM-DD_hh-mm-ss", timestamp())
  root_dir = abspath("../src")
}

data "archive_file" "archived_source_code" {
  type        = "zip"
  source_dir  = local.root_dir
  output_path = "/tmp/pubsub-triggered-code-${local.timestamp}.zip"
}

resource "google_storage_bucket" "code_archive_bucket" {
  name     = "pubsub-triggered-code-archive"
  location = "EU"
  storage_class = "ARCHIVE"
  uniform_bucket_level_access = true
  lifecycle_rule {
    condition {
      age  = 367
    }
    action {
      type = "Delete"
    }
  }
}

resource "google_storage_bucket_object" "archive" {
  name   = "pubsub-triggered-code-${local.timestamp}.zip"
  bucket = google_storage_bucket.code_archive_bucket.name
  source = data.archive_file.archived_source_code.output_path
}

--- EDIT #2 ---

After manual modification of the variable in the GUI of GCP and running terraform plan, I've been warned by Terraform that variable has been manually changed and it's presenting new variable name.

Note: Objects have changed outside of Terraform
Terraform detected the following changes made outside of Terraform since the last "terraform apply":
  - "DOMAIN"     = "TEST" -> null
  + "APP_DOMAIN" = "TEST"

--- EDIT #3 ---

After manual change mentioned in previous edit it's working. I remember I was having this issue before, so it's likely to happen in the future, really close one. This is the second time I'm using this code and second time I was having this issue.

--- EDIT #4 ---

Now the code does not update at all. I have to do some manual actions, like remove the function to see changes. This was the official example presented in the Terraform docs.