Your sitemaps are probably lying to you.
Not about which URLs exist — about what's actually happening with indexing.
Most sitemap setups split files arbitrarily: sitemap_0.xml, sitemap_1.xml, sitemap_2.xml. Nice and tidy. Completely useless for diagnostics.
When indexing drops, you see a number go down in Search Console. But which pages? Products? Articles? Location pages? With arbitrary splits, you genuinely cannot tell.
Segment by content type instead, and sitemaps become a monitoring system. A drop in your products sitemap means something very different from a drop in your articles sitemap, and you can act on it immediately.
There's also a detail most people miss: Google Search Console caps issue sample data at 1,000 URLs per sitemap. One monolithic sitemap = 1,000 samples. Ten segmented sitemaps = up to 10,000. That's not a minor difference when you're troubleshooting at scale.
Wrote up the full approach: segmentation strategies, hierarchical indexes, naming conventions, and the trade-offs of making your sitemap structure public:
visively.com/kb/algorithms/x…