Changing the Navigation Label does not affect how it appears on website

When I change the Navigation Label under Appearance/Themes, it does not affect my menu labels. These labels are always the same as my post title. I’ve had to keep my post titles very short for the time being so that my website doesn’t look cluttered with long menu options. This started to be an issue randomly a couple of weeks ago. I tried disabling all of my plugins but that didn’t solve the problem.