taxonomy not shown in admin panel after saving

I created a custom `page.collection` for a listing page containing some products. After that, I created the respective blueprint file. Everything is perfectly fine, until I try to edit and save the taxonomy categories from the admin panel.

The markdown file is filled with the correct input, but after reloading the admin panel, the previously filled category field is empty.

This is my `productlist.md`

```
---
kollektion:
-
title: Umbau
'@taxonomy':
category:
- umbau
routable: false
visible: false
title: Produkte
content:
items: '@self.children'
order:
by: date
dir: desc
---
```
This is my blueprint `productlist.yaml`

```
title: Seite
'@extends':
type: default
context: blueprints://pages

form:
fields:
tabs:
type: tabs
active: 1

fields:
content:
unset@: true
type: tab
title: Seite
fields:

header.headline:
type: text
label: Seitenüberschrift

header.kollektion:
name: kollektion
type: list
style: vertical
label: Kollektion
fields:
.title:
type: text
label: Title
.@taxonomy:
type: taxonomy
label: PLUGIN_ADMIN.TAXONOMY
multiple: true
validate:
type: array

route:
type: select
label: PLUGIN_ADMIN.PARENT
classes: fancy
'@data-options': '\Grav\Common\Page\Pages::parents'
'@data-default': '\Grav\Plugin\admin::route'
options:
'/': PLUGIN_ADMIN.DEFAULT_OPTION_ROOTs
advanced:
unset@: true
```

As you can see, after saving from the admin panel, the markdown file is filled correctly.
But the admin panel stays empty.