URGENT: My Free XML Sitemap Generator is Causing Critical Indexing Issues – What Am I Missing?

I am absolutely tearing my hair out trying to fix a critical issue with our Free XML Sitemap Generator tool. For the past week, it's been behaving erratically, consistently failing sitemap validation for numerous client sites. This isn't just an annoyance; I'm seeing direct evidence of drops in crawl rates and indexed pages, which strongly suggests that these validation failures are directly impacting our clients' search engine visibility and indexing.

Our tool's primary purpose is to generate clean, valid XML sitemaps for our clients' websites. It's been a stable part of our offering for a long time. The only recent changes were some minor dependency updates in the Node.js backend and a migration to a slightly newer Nginx version on our server. I can't pinpoint anything specific that would cause such a dramatic shift in behavior.

The problem manifests in several ways. Google Search Console is reporting errors like “Invalid XML format” or “Malformed URL” for sitemaps that were previously fine. Other online validators (like XML-Sitemaps.com’s own validator) are also flagging issues like “Unexpected character at line X, column Y” or even “Duplicate URLs detected” within the generated XML, even when the source data clearly doesn't have duplicates. This is affecting a significant number of our clients, and their indexing performance is taking a hit, which is incredibly frustrating.

I've spent countless hours on this, trying everything I can think of:

• Manual Validation: I've manually validated dozens of generated sitemaps using Google Search Console's sitemap tester and various third-party online XML validators. They consistently fail, often with different but equally unhelpful error messages each time.
• Code Review: I've meticulously reviewed the Node.js backend logic responsible for sitemap generation, checking for obvious syntax errors, potential infinite loops, incorrect XML encoding, or file output issues. Everything looks correct on the surface, and no recent code changes directly touch the XML serialization logic.
• Server Logs Check: I've scoured the Nginx and Node.js application logs for any related errors, memory limit warnings, or timeout issues during the sitemap generation process. There are no clear culprits, just standard access logs.
• Database Integrity: I've verified the database where URLs are pulled from for any corruption, malformed entries, or encoding problems. The database seems perfectly fine, and the raw URL data is clean.
• Dependency Updates/Rollbacks: I've tried both updating and rolling back the minor Node.js dependencies that were recently touched, but neither action had any impact on the sitemap generation errors.

Has anyone experienced similar critical indexing issues with their custom XML sitemap generators recently? Are there any obscure XML schema validation rules or server-side configurations (e.g., mod_deflate, Content-Type headers, or character encodings) I might be overlooking that could cause this? Could this be a subtle caching problem, where an an old, malformed sitemap is being served despite a new, correct generation attempt? What advanced debugging techniques or tools would you recommend for pinpointing such elusive sitemap generation errors?