TailwindCss’s mobile breakpoints doesn’t work

I am building a custom navbar in React using TailwindCss for styling. For reference I am following a tutorial on YouTube which’s code can be found here.

When I use the mobile breakpoints as prefix like in the example below only the div saying “mobile” works as it should be:

<nav>
  <div className={"flex"}>
    <div className={"hidden md:flex"}>desktop</div>
    <div className={"md:hidden"}>mobile</div>
  </div>
</nav>

The output “desktop” should be shown when the text “mobile” vanishes but for some reason it doesn’t. Am I doing something wrong or is there a problem with Tailwind?

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