How to stop pretty permalink to redirect automatically?

I got hit the second time:

So someone on google search our site, got some URL like https://our.fqdn.com/release-doc/xxx-yyy.pdf.

And some security analyst removed the xxx-yyy.pdf, and WP suddenly https://our.fqdn.com/release-doc/ automatically rewrite to https://our.fqdn.com/some-other-location/some-other-unrelated-page/, and that page or post is not supposed to be public on our site.

Why? and how to fix it?