Preview: Highlight Shortcut underlines my text instead

Hi! This question is in regard to a problem I am facing with the preview application.

When I use the highlight shortcut (^ + ⌘ + H) or Annotate > Highlight Text method, my text ends up being underlined instead of highlighting. The highlight still works if I use the highlighting function on the top right hand area of the application.

I'm currently using High Sierra. Has anyone faced this problem before and knows of a solution to make the shortcut work again?

Thank you!