Why is the hreflang for the Help Center showing only "en" and not "en-US"?
If the default language of your Help Center is set to English, all requests are redirected to the locale defined as the default one for English - which in is defined as en-us.
As per Google's guidelines:
It's a good idea to provide a generic URL for geographically unspecified users if you have several alternate URLs targeted at users with the same language, but in different locales. For example, you may have specific URLs for English speakers in Ireland (en-ie), Canada (en-ca), and Australia (en-au), but want all other English speakers to see your generic English (en) page, and everyone else to see the homepage. In this case you should specify the generic English-language (en) page for searchers in, say, the UK. You can annotate this cluster of pages using a Sitemap file or using HTML link tags like this:
<link rel="alternate" href="http://example.com/en-ie" hreflang="en-ie" />
<link rel="alternate" href="http://example.com/en-ca" hreflang="en-ca" />
<link rel="alternate" href="http://example.com/en-au" hreflang="en-au" />
<link rel="alternate" href="http://example.com/en" hreflang="en" />
Therefore, if there's only one locale defined for your Help Center - i.e. https://subdomain.zendesk.com/hc/en-us and to comply with Google's guidelines, the hreflang tag will be defined as:
<link rel="alternate" hreflang="en" href="https://subdomain.zendesk.com/hc/en-us" />