Sudden increase in "Crawled ‐ currently not indexed" URLs

As per the title of the thread: suddenly a number of URLs that were previously indexed are now marked in the Google Search Console as "Crawled - currently not indexed".

The content is unique and good quality (over 1,000 words, with videos, images and proper internal link).

It started right after the September 12 Core Update and affected around 50% of my site's URLs.

Does any of you have experienced something similar?

No one? My guess is that the penalty (if this is the case, due to the fact that the de-indexing coincides with the launch of the algorithm update) it is due to the way I have implemented structured data.

Instead of using a plugin, I added the structured data JSON via a / html block at the end of the post. In almost all the posts I use structured data like FAQ or How To, so part of the content of the posts is "copied" in the JSON of the structured data (questions / answers, steps to explain how to do a certain thing).

Could it be this the issue? Maybe what the algorithm targeted is duplicate content within the post? If so, how would you fix it?

My second guess is that simply Google does't like PAA sites anymore.