Adding default tags to resource in custom terraform provider

Terraform Providers
1

I am writing a custom terraform provider for an internal API.
For any resource that are created by the provider I would like to add a tag to the resource of the form: Created-By-Terraform: True along with other tags that the user providers.

return &schema.Resource{
.....
"tags": {
				Type:        schema.TypeMap,
				Required:    false,
				Optional:    true,
				Computed: true,
				ForceNew: false,
				Description: "Tags (key = value pairs)",
			},
....

func GetTfTags(d *schema.ResourceData) []string {
	tfTags := d.Get("tags").(map[string]interface{})
	tags := []string{"Created-By-Terraform:true"}
	for k, v := range tfTags {
		tags = append(tags, k+":"+v.(string))
	}
	return tags
}


func resourceMyResourceCreate(d *schema.ResourceData, m interface{}) error {
    tags := GetTfTags(d)
   // create the resource on the backend
}

func resourceMyResourceUpdate(d *schema.ResourceData, m interface{}) error {
    tags := GetTfTags(d)
   // Update the resource on the backend
}

This works perfectly fine as expected. However, acceptance tests keeps failing when I try to provide a user-defined tag “foo”: “bar”

Step 0 error: After applying this step, the plan was not empty:
DIFF:

        UPDATE: my_resource.my_resource
            tags.Created-By-Terraform: "true" => ""
            tags.foo:                  "bar" => "bar"

STATE:
          tags.Created-By-Terraform = true
          tags.foo = bar

Any idea why this is the case? Do I need to use customdiff functionality?
I don’t want users to set lifecycle at provider level or at each resource level when they write HCL.