amp-subscription: Entitlement failed for local / No messaging channel errors

### Description

We are noticing that the Google News App (iOS) does NOT call our auth / pingback service endpoints and existing users have to click the login button on every article to bypass. Have you had or are you having a similar issue with your implementation?
I believe I have tracked down the cause of the issue but do not have a solution. The issue is related to the “#webview=1” parameter that amp-subscription adds. The app/browser is supposed to create a messaging channel to call the service endpoints but fails to.

### Reproduction Steps

-Launch page in Chrome... (URL that Google News App uses.)
https://www-dallasnews-com.cdn.ampproject.org/v/s/www.dallasnews.com/business/autos/2021/05/15/car-shopping-isnt-likely-to-get-easier-as-chip-shortages-deplete-auto-dealers-lots/?outputType=amp&_js_v=0.1#webview=1&cap=swipe
- Refresh (first view is not metered)

### Relevant Logs

```shell
Chrome Console log...
error.js:59 Uncaught (in promise) Error: fetch entitlements failed for local: timeout
at ya (error.js:59)
at wb.g.createError (log.js:365)
at timer-impl.js:145
at timer-impl.js:86
error.js:59 Uncaught (in promise) Error: No messaging channel: initMessagingChannel timeout
at ya (error.js:59)
at wb.g.createError (log.js:365)
at timer-impl.js:145
at timer-impl.js:86
```

### Browser(s) Affected

Chrome

### OS(s) Affected

IOS

### Device(s) Affected

_No response_

### AMP Version Affected

_No response_