Vue - v-if not updating with onMounted

I'm simply trying to make a div only appear once a variable changes from false to true and I'm telling it to change the value once the component is mounted. However the v-if which is supposed to be looking for that change is not working to create the div.

<script setup>
  import { onMounted } from 'vue'
let onceMounted = false
onMounted(() => {
onceMounted = true
console.log(onceMounted)
});

<div>
  Hello
  <div v-if="onceMounted === true">
    World
  </div>
</div>

The console log confirms it changing fine, just the v-if isn't updating. I've checked using v-show after checking out other questions on here like this one. I'm guessing there's something I'm just missing?