Markdown blocks are not sending with incoming webhooks

Hey folks, I'm trying to send a message with some markdown in it, but its not being honored.

According to the reference these should work?
https://api.slack.com/reference/block-kit/blocks#markdown

Here's my example with inline blocks

- name: Announce Release
    uses: slackapi/slack-github-action@v2.1.0
    with:
      webhook: ${{ secrets.MY_SECRET }}
      webhook-type: incoming-webhook
      errors: true
      payload: |
        text: "New Release: Shared Helm Chart: 1.2.3"
        blocks:
          - type: header
            text:
              type: plain_text
              text: ":tada: New Release: Shared Helm Chart: 1.2.3"
          - type: divider
          - type: section
            text:
              type: mrkdwn
              text: "## :notebook: Release notes"
          - type: section
            text:
              type: mrkdwn
              text: Blah
          - type: divider
          - type: section
            text:
              type: mrkdwn
              text: "** :computer: Release URL**"
          - type: section
            text:
              type: mrkdwn
              text: http://my_url.com

And this is what it ends up looking like:

I have also tried with a payload file:
GHA

- name: Announce Release
      if: github.event_name == 'push' && github.ref == 'refs/heads/slack-releases3'
      uses: slackapi/slack-github-action@v2.1.0
      with:
        payload-file-path: .github/workflows/payload.json
        webhook: ${{ secrets.MY_SECRET }}
        webhook-type: incoming-webhook
        errors: true

JSON

{
  "channel": "${{ env.SLACK_CHANNEL_ID }}",
  "text": "Messages met made meetings meet",
  "blocks": [
    {
      "type": "section",
      "text": {
        "type": "mrkdwn",
        "text": "# Slack notifications sound following the most recent commit pushed"
      }
    }
  ]
}

Result

I would expect to be able to use bold and

HEADER

and other markdown fun in a block called mrkdwn.

Am I doing something wrong?

Start with the inline YAML example and .github/workflows/payload.json, then read the linked Slack Block Kit mrkdwn reference. Reproduce the incoming-webhook request with both payload forms and trace how the action handles the blocks; done means the expected formatting behavior is confirmed and covered by an appropriate test or documented limitation.