Again anecdotal, but I lost the majority of my SEO traffic late last year around the time of a core update. I've spent the best part of a year attempting to repair it, on the assumption I'd committed some heinous SEO crime. The more time that passes, I'm starting to think that the issue isn't mine so much as Google's. It's baffling. I wrote about it here:

https://johnnyreilly.com/how-i-ruined-my-seo