terraform overwrite tag value in sub modules

I'm using terraform modules and passing a set of default_tags to sub-modules for consistent tagging. This is shown below, and has the advantage that sub-modules inherit their parent's tags, but can also add their own.

However what I'd also like to do is to be able to overwrite some of the inherited tags values, especially "Name". But I can't seem to make this work.

In the example below (terraform 13.5 with AWS) the first value specified for any tag, e.g. "scratch-test" in the root module is cascaded down to the sub-modules, and cannot be changed. So both the VPC Name tag and the subnet Name tag = "scratch-test".

How can I overwrite a tag value in a sub-module?

# root variables.tf
variable "default_tags" {
    type = map
    default = {
        environment_type = "Dev Environment"
    }
} 


# root main.tf
provider "aws" {
    region = var.region
}

module "vpc" {
    source = "../../../../scratch/tags-pattern"
    vpc_name                = "scratch-test"
    public_subnets          = ["10.0.0.0/26"]

    default_tags = merge(map(
        "Name", "scratch-test"
    ), var.default_tags)
}

# ../../../../scratch/tags-pattern/main.tf
module "the_vpc" {
    source = "../../terraform/tf-lib/network/vpc"

    vpc_name = var.vpc_name
    vpc_cidr = "10.0.0.0/24"

    default_tags = merge(map(
        "Name", "scratch-test-vpc",
        "vpc_tag", "vpc"
    ), var.default_tags)
}

# Add a subnet
module "public_subnets" {
    source = "../../terraform/tf-lib/network/subnet"
    vpc_id                  = module.the_vpc.output_vpc_id
    subnets                 = var.public_subnets
    default_tags = merge(map(
        "Name", "scratch-test-subnet",
        "subnet_tag", "public"
    ), var.default_tags)
}

# tf-lib/network/vpc/main.tf
resource "aws_vpc" "vpc" {
    cidr_block = var.vpc_cidr
    enable_dns_support = true
    enable_dns_hostnames = true
    tags = merge(map(
        "module", "tf-lib/network/vpc"
    ), var.default_tags)
} 

The variable.tf file for each module contains the statement:

variable "default_tags" {}