Google has officially announced webmasters can't do anything to fix this #*$!
In fact you may have done absolutely nothing wrong and there is no magical "fix" for good sites, quite simply you may just have to roll with the dice and accept it for what it is.
Alternatively, if you have plenty of spare time available, you could examine in-depth the sites/pages which are now ranking above you and do what they're doing however, if one's pages are that good what's the point in changing them because when the next big update comes along you may lose out again and one's original pages may return?
This is a true story. Just over a year ago one of my sites that had been #1 for many years for my industry's #1 keyword widget product term simply disappeared, it didn't matter how far I went down the SERPs, it had gone. Now bear in mind not only do I own the keyword domain, I hold the global trademark for it, I introduced it to the global market almost 40 years ago, everything about this specific widget success within my industry was down to me.
Interestingly Quora who ranked for this widget on the first page actually cited me in their first paragraph as THE informational and image source for this widget. A couple of months ago a new #1 appeared in the SERPs from a very large US corporation with a 2018 date and, basically, they had badly re-written my information however it was a very, very poor attempt.
Meanwhile I did nothing, our sales were fine and the site was #1 on ALL other search engines then, out of nowhere, two weeks ago my site suddenly reappeared back on the first page and is currently floating between 1st and 5th.
Don't ask me how or why, I did NOTHING, I've seen this happen several times over many years on Google with sites and, more often, images. I assume it's a glitch/bug that they have no idea exists or have any idea of how to resolve it.
The thing is, unless one is a HUGE company and Google really loves you, it is very unlikely that any of us can do anything about what they have done.
Devil's Advocate Time And Not Aimed At Anyone Specific: How many of you are absolutely 100% sure that your pages are THE best for your widgets or how many of you have been extremely lucky with your rankings and traffic until now and have been caught out by this/any update?