Google’s John Mueller said on Twitter that if you cannot have a clean header in your source code where Google can fully interpret the hreflang, then you are out of luck. Google said either fix the issue with your header or you really cannot depend on Google properly understanding your hreflang markup.
The issue came up with Akamai iframe broke the header and resulted in Google having issues processing the hreflnag. This was from Barry Adams:

Yes and yes. The Akamai iframe broke Google’s processing of the site’s hreflang tags. Definitely an issue.
— Barry Adams 🧩 (@badams) March 12, 2019
John Mueller from Google chimed in to say they make no exceptions here, fix the issue or you are out of luck:

We don’t have exceptions for sites that “can’t make a clean head section”, so you either have to fix it (maybe move it to the bottom of the head?), use a different method (headers, sitemap), or assume we won’t always be able to pick it up.
— 🍌 John 🍌 (@JohnMu) March 12, 2019
Forum discussion at Twitter.

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