My site has been doing very well in recent months. However, after updating our main page about three weeks ago (it hadn't been touched for several months) it immediately wobbled and then dropped in rankings while a couple of competitors went up. It used to have a Moz score in the 70s, multiple H1 tags and all kinds of other SEO technical fails. We updated it so that both Yoast and Moz love it and we're using the same optimisations that have served us well in 100 other posts. (Moz says there might be keyword stuffing but it always seems to say that no matter how much I reduce mentions - there are fewer mentions than there were before anyway).
Did something happen with Google? I see that there were some major Core test changes happening around then but it seems weird that this would be the exact same time as we optimised this page.
I've spent the past two weeks A/B testing and I cannot believe it's content related.