It doesn't really work as expected on the desktop either.
On the web, homepage typically means the page that's displayed on the root domain url, kbin.social, for example.
I think the expectation is that setting the homepage to subscribed means that the root domain page should only show threads from subscribed communities
That's wierd. If auto ration is disabled at the Android system level I don't think it's possible for an app or a web page to overide that. Are you sure auto rotation is off in Settings or Quick settings?