hreflang links in HTTP headers are only present in localhost

[empz]

I have a project that follows the approach of the following example app:
https://github.com/tolgee/next-app-example
It's pretty much the same but with localized pathnames.

In localhost the hreflag links in the HTTP header works just fine. Both during next dev as well as running a production build.

But when I deploy it to Vercel, they are gone.

I see your app router example app is also deployed to Vercel and the headers are included.

What could be the cause of mine not being included? Any ideas what might be removing them?