Nuxt, transitions not working for child routes (displayed using NuxtChild)

Question

I'm having trouble getting transitions to work for my child routes.

I have the following pages:

pages/
  child/
    _id.vue
  child.vue
  index.vue

Navigating between index and any of the child routes triggers transitions, but when navigating from one child route to another child route there is no transition.

Note that there is a static route /child and dynamic routes for /child/_id. The <nuxt-child /> component is inside child.vue:

<template>
  <div class="container">
    <h1 class="title">
      Child Root Page
    </h1>
    <nuxt-child />
  </div>
</template>

<script>
export default {
  key(route) {
    return route.fullPath
  },
}
</script>

If I remove the static route child.vue and move <nuxt-child /> to index.vue all transitions work, but then I don't have the 'parent' child page anymore.

Is it possible to get the transitions to work in this case?

I have a small repo showing the problem and a github page with the site deployed.