When legacy pages suddenly reappear in search results, it usually points to...

https://stateofseo.com/should-i-301-redirect-old-urls-or-leave-a-404-the-reality-of-content-decay/

When legacy pages suddenly reappear in search results, it usually points to neglected server caches or forgotten subdirectories being indexed. This creates a confusing experience for users and dilutes your domain authority