Custom nextgen-editor field renders page image url incorrectly

I have a custom blueprint with the default editor set to nextgen-editor, and a second custom nextgen-editor field, defined as `additional`. Both fields are working normally except that for the `additional` field a) there is no way to insert a page image, and b) if the page image is copied in from the default field, it renders incorrectly.

The resulting markdown is identical in both (``) but the rendered HTML for the `additional` field is incorrect.

Here's the page header and md content:

```
---
title: 'Climate Forum'
additional: "**UPCOMING CLIMATE FORUM**\n\nCooperative Climate Solutions | Thursday March 17th 2022 | [RSVP here](https://us02web.zoom.us/meeting/register/tZIvf-yorT4qHN3R2tGLXhMTDgLvlmvfsKoL)\n\n\n\nThanks for choosing to act:\n\n**Leigh Grigg** - The Sustainability Project\n\n**Joachim Ostertag** - Climate Action Team Owen Sound\n\n**Jenny Carver** - Georgian Bluffs Climate Action Team\n"
media_order: 'website.png,thumbnail_mar17_poster.jpg'
---

The goal of Climate Forum talks are to learn together the opportunities, challenges and barriers to climate action in our region.

```

And here is the Twig, contained in a Gantry5 Custom HTML particle, that renders the additional field:

```
{% if grav.page.header.additional %}
{{ grav.page.header.additional|markdown|raw }}
{% endif %}
```

Here are the rendered img outputs for the two fields:

## Default field

```

```

## Custom field (`additional`)

```

```