Browser calls app to access website

I use the BBC News app now and again. On looking at battery usage recently it appeared that the app was using the battery (calling home?) even when I wasn't using it. So I disabled access to both Wi-Fi and Mobile Data.
Today I wanted to get some info on a news item I'd heard on the radio so opened Firefox and used Startpage to find some links. One of them was a BBC news page. So I clicked on it and Firefox showed that the page wasn't available:
ERR_NETWORK_ACCESS_DENIED
I tried with Vivaldi and got the same result. Other webpages at newspaper sites all worked perfectly with either browser. Intrigued, I cleared out all history and cookies, re-enabled the BBC News app access to mobile data, and did the same searches for the news story with Firefox and Vivaldi. On clicking on the BBC news link, the pages now appeared without problem.
Can anyone confirm this happens? If so, why?
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Jeff