Update field in lifecycle ignore_changes when there are other changes

Terraform
1

I’m trying to update my Lambda function’s description with a variable that changes on every run (I’m using part of TFC_CONFIGURATION_VERSION_GIT_COMMIT_SHA). I only want the Lambda function to update when there are changes to fields that are not the description but when those other fields do change, I want the description to be updated. I thought this was how it worked but my descriptions are not getting updated when the code changes.

This is what my Lambda definition looks like:

resource aws_lambda_function cloudfront_errors {
  function_name    = "cloudfront-errors"
  description      = "TFC Build (${var.tfc_git_hash})"
  runtime          = "nodejs12.x"
  role             = aws_iam_role.cloudfront_errors.arn
  handler          = "index.handler"
  filename         = data.archive_file.cloudfront_errors.output_path
  source_code_hash = data.archive_file.cloudfront_errors.output_base64sha256
  publish          = true

  lifecycle {
    ignore_changes = [
      description]
  }
}

If I remove the ignore_changes block, the Lambda function updates on every plan. If I leave it in, the description value never changes (even though the value of var.tfc_git_hash does change.

This lives in the “nice to have” world but the behavior goes against what I was expecting. Any ideas how to achieve what I’m trying to do?