Google AMP : `Method: ampUrls.batchGet` is throwing `NO_AMP_URL` even if the AMP is present

Sample webpage: https://www.bloombergquint.com/politics/jyotiraditya-scindia-meets-prime-minister-even-as-congress-says-he-remains-incommunicado

If you check the source code you can see that AMP is present. The source code and Google AMP Test also confirm the same.

Source code:

Google AMP Testing tool

But the PSI api call gives me this error:

{
  "urlErrors": [
    {
      "errorCode": "NO_AMP_URL",
      "errorMessage": "No AMP URL for the request URL.",
      "originalUrl": "https://www.bloombergquint.com/politics/jyotiraditya-scindia-meets-prime-minister-even-as-congress-says-he-remains-incommunicado"
    }
  ]
}

Findings when debugging:

1. The service worker is deleting the rel="amphtml" snippet from the page source after JS is powered up. (Check the source code after page is loaded completely in view page source)
2. The mainEntityofPage structured data is set to the home page of the site (bloombergquint.com) instead of the story page. (bloombergquint.com/:story-slug)

3. ThumbnailURL is missing from structured data(SD) but in the preview is working fine with SD testing tool. (In preview the page is shown as AMP also.)

4. What are the parameters this API is checking. (API docs)

5. If the API is a success, the PSI (pagespeed insights) will hopefully show the AMP Url discovered message?

Edit

If the source code has an absolute URL to the AMP page than a relative URL, it shows success. What's the recommended content for rel="amphtml?