Search results home page title not showing properly but other pages do?
Could the Google account holder for the domain have set the listing to use a different value than what's in the title element?

Craft doesn't do anything like this out of the box and the SEO plugins that would change the title value you'd see that in the DOM. Is there a meta tag in the head maybe that Google reads to override the title element?

There's no default front-end templating in Craft CMS so whatever is doing this is most likely either whatever the developer's added to the homepage or layout template or Google itself.