Latest Blog Post Appearing as Title of 404 Error Page

The issue I’m experiencing is identical to the user here: Incorrect page title sometimes displays on X - Custom 404 page [Follow-up 2]

When I go to an invalid page, the title of the latest blog post “Non-Negotioables for Space: Testing Optics…” appears in the tab (see below):
https://optikos2stg.wpenginepowered.com/thisisnotarealpage

The actual Page Not Found page title should be “Page Not Found - Optikos”:
https://optikos2stg.wpenginepowered.com/page-not-found

What can I do to resolve this?