Gary Illyes from Google wrote in a Reddit thread that “generally” your hreflang annotations are not processed or picked up until all the pages in the set that you are linking to are crawled and re-indexed. He wrote “Generally the hreflang annotations are picked up once all pages that you linked together with them are crawled and (re)indexed.”
He explained “So if you have a topic in English and you have page A, B, and C for it that are targeting different languages (and potentially countries), Google would have to crawl and (re)index at least two pages for it to start to work, and all three for the full effect.”
He said he knows this because he “worked on Google’s hreflang support implementation.”
This makes sense, seems logical, but good to hear it be said – although, maybe Google said this before and I missed it.
Forum discussion at Reddit.