Massive Drop In Indexed Pages - Everything Getting Stuck In 'crawled - Currently Not Indexed' (no Recent Changes, Protections Removed)

Hey everyone,

I'm struggling with a serious indexing issue that keeps repeating across several of my business websites that have over 10 million pages. (Big e-commerce projects that are custom-made).

Here’s what’s happening:

• The site had ~2.5 million indexed pages (others are duplicates that have either redirects or selected as non-canonicals), but now it’s down to ~690k. (the drop is happening not in one day, but like losing ~300-400k indexed pages every 3-4 days). The problem is affecting all my projects, but not so dramatically.
• Most of the dropped URLs are showing in Google Search Console as “Crawled – currently not indexed.”
• We haven’t made any significant updates or content changes recently. (Because there is not match, we can update - it is an automotive parts site, their main information is part name, OEM/part number, and diagrams).
• All anti-DDoS (Cloudflare) and security protections were disabled/removed, just to rule out crawling or blocking issues.
• No “noindex” tags, no major canonical or robots.txt issues detected. (We are using a self-referencing strategy for product pages).
• The problem keeps repeating periodically — sometimes one site gets hit, sometimes another, always the same pattern: pages move from indexed → crawled but not indexed.

Has anyone experienced a similar pattern or found what triggers this?
Could this be related to site quality signals, server response timing, crawl budget, or something algorithmic on Google’s side?

It feels completely lost. This issue has a severe impact on the business I represent, significantly affecting overall traffic and profitability. Projects are old and have a pretty big crawl budget (over 1tb crawled every month if GSC info is correct).

Any advanced insights or ideas would be appreciated.
Thanks in advance to anyone who takes the time to help me out!