The only thing I can find are quotes from Mueller - implying that it doesn't really matter for SEO rankings, as to how you organise your heading tags. What's more important is that if Google understands your content, then it can present it better to users.
So… this question of… how should I order my H1, H2, H3, headings and what should the content be, that’s something from my point of view isn’t really that relevant.
So it’s not so much that suddenly your page ranks higher because you have those keywords there. But suddenly it’s more well Google understands my content a little bit better and therefore it can send users who are explicitly looking for my content a little bit more towards my page.
But it’s not that suddenly your page will rank number one for competitive queries just because you’re making it very easy for Google to understand your content.
All from Search Engine Journal