Renaming the Error/404 Page

Asking here before I submit this as a bug.

I’ve created a template for my 404 page (as type “Error” template).

When testing the finished templated, I noticed that it names the page as “Page not found”.
EG: mydomain.com/oidjfogjdfg (or other gibberish, non-real page slug).

I’ve gone into the template’s Page settings > SEO and set the ideal page name, but no matter what I do, it remains as “Page not found”.

There is no interference from any 3rd party plugins, etc.

Am I doing this wrong, or is this a bug?