I am getting traffic to unknown URL like 'www.xyz.com/amp-helper-frame.html/?appId='. I don't know how this URL is getting triggered. Could not even find a source on web.
Found the issue.
This is due to the missing 2 files that need to be added to amp onesignal. Here are the 2 files that need to be added to the root of your site 1) amp-helper-iframe.html 2) amp-permission-dialog.html
you can find the same in onesignal official documentation (point number 3). Link: https://documentation.onesignal.com/docs/amp-web-push-setup#section-3-configure-the-amp-web-push-extension