Directus Flow Issue: Field Undefined at Final Update Data Stage

Question

I'm encountering an issue with a Directus Flow where my plain, nullable full_slug field is being set to undefined in the Update Data stage, despite being correctly defined after the Custom Script execution.

The flow consists of the following stages:

1. Event Hook (Trigger: pages.items.update)
2. Read Data (Parent Item)
3. Read Data (Current Item)
4. Custom Script
5. Update Data

Here's the Custom Script that correctly updates the full_slug:

module.exports = async function(data) {
  const slug = data.$trigger.payload.slug;
  const parentItem = data.item_read_lhmzf;
  const currentItem = data.item_read_mn69m;

  if (parentItem && parentItem.full_slug) {
    data.$trigger.payload.full_slug = parentItem.full_slug + '/' + slug;
  } else if (currentItem && currentItem.full_slug) {
    const parts = currentItem.full_slug.split('/');
    parts[parts.length - 1] = slug;
    data.$trigger.payload.full_slug = parts.join('/');
  } else {
    data.$trigger.payload.full_slug = slug;
  }

  return data;
}

Event payload slug: rabbit-facts Parent item full_slug: home/blog Current item full_slug: home/blog/rabbit-factoid

After the Custom Script execution, full_slug is correctly set to home/blog/rabbit-facts.

At the Update Data stage, the ID is set to:

{{$trigger.keys[0]}}

…which seems to be correct.

And the payload is:

{
    "full_slug": "{{$trigger.payload.full_slug}}"
}

However, the final output in the Update Data stage shows:

{
  "collection": "pages",
  "payload": {
    "full_slug": "undefined"
  },
  "key": [
    "b7034f18-8079-43aa-a448-5c3d45c71cbe"
  ]
}

Any insights would be greatly appreciated. Thanks in advance for your help!