Apparently the problem comes from the page.data and seems to be a bug (posted here and here.

Using data.personalMessage message instead of page.data.personalMessage is both a workaround and a better practice.

//+page.svelte
<script>
   const { data } = $props();
</script>
<TypewriterEffect>
   {@html data.personalMessage}
</TypewriterEffect>

Thanks to @brunnerh for the solution.