It is common for a fix in one part of a site, such as a shared component update, to unintentionally break accessibility elsewhere, for example a global navigation change that removes a working skip link, or a design system update that quietly drops a focus indicator style across every button on the site.
Mature accessibility programs build automated accessibility checks into their continuous integration pipeline, so a scanner runs on every pull request and flags regressions before they reach production, rather than relying solely on periodic full-site audits to catch them well after the fact.