Title in browser tab shows URL

Hello,

The titles in the browser tabs don’t display correctly. When loading, the wording is right but eventually, the webpage URL is displayed in the browser tab.

I’ve tested this with Chrome, Safari and Firefox, the issue doesn’t change.
I’ve changed the settings in wordpress’ Settings->General, the issue doesn’t change.

The only time the issue disappeared is when I switched back to the Twenty Fifteen theme. When I activated the Highend Theme again, the issue appears again (both on the regular and child theme).