Header title is displaying site name
Once you add titles with Yoast SEO plugin, header will display that title and site name, instead of the page title.
In order to show only title, without site name, you could change page title template to display only the title. In Yoast > Titles & Metas > Pages Title template, you could add title template as:
Other option would be to switch to All in One SEO Pack plugin, that does not affect header title.