{{ dirname }} placeholder not set for `public_folder` when initially creating a entry in a nested collection

[Freddo3000]

Describe the bug

When creating an entry in a nested collection, the {{ dirname }} placeholder appears to not be set for public_folder.

Using the following collection:

  - label: Dokumentmappar
    label_singular: Dokumentmapp
    name: documents
    folder: src/dokument/
    create: true
    nested:
      summary: "{{ fields.title }}"
      depth: 10
    fields:
      - {label: Titel, name: title}
      - {label: Anteckningar, name: body, widget: markdown, required: false}
      - {name: permalink, widget: hidden, default: false}
      - label: Dokument
        name: files
        widget: list
        summary: "{{ fields.file }}"
        field:
          name: file
          widget: file
          media_folder: "/src/dokument/{{ dirname }}"
          public_folder: "/dokument/{{ dirname }}"

this causes the file paths for public_folder to get improperly set, despite the files getting placed correctly by the media_folder attribute.

To Reproduce

1. Use the above collection configuration.
2. Create a nested entry with files attached.
3. Notice that the filepaths set in the .md file is /dokument/filename.ext instead of /dokument/nested/filename.ext
4. Notice that the files are in the correct position, and editing the nested entry yields the correct paths.

Expected behavior

Files created should have their filepaths correctly set.

Screenshots

n/a

Applicable Versions:

• Decap CMS version: 3.10.1
• Git provider: GitLab
• OS: Ubuntu
• Browser version: Mozilla Firefox 148.0
• Node.JS version: v22.22.1

CMS configuration

https://git.ludd.ltu.se/lulespexet/www/-/blob/master/src/admin/config.yml

Additional context

Example commit: https://git.ludd.ltu.se/lulespexet/www/-/commit/ba5b2f6ae8cd03081fe3ab21ecd813f911d7dda5