Getting the Source AMI id when using HCL configuration

Packer
7

So I tried to use the tag instead of the tags block so I can do a dynamic block with other fixed tags.

However, it looks like {{ .SourceAMI }} would return no value:

  dynamic "tag" {
    for_each = merge(
      var.my__other_tags,
      {
        Name       = "${var.ami_name} {{ isotime \"2006-01-02-15-04\" }}",
        source-ami = "{{ .SourceAMI }}"
      }
    )
    content {
      key  = tag.key
      value = tag.value
    }
  }

From the packer logs:

==> amazon-ebs.amzn2: Creating AMI tags
    amazon-ebs.amzn2: Adding tag: "Name": "Amazon Linux 2 AMI 2020-10-20-16-48"
    amazon-ebs.amzn2: Adding tag: "source-ami": "<no value>"
...

I’m using the latest Packer v1.6.4. Any ideas what I did wrong here?