SelectBoxEditor multiple selections with static values not working

Using the following example of the documentation and add multiple: true ends up in a unexpected behaviour:

targetMode:
  type: string
  defaultValue: 'firstChildNode'
  ui:
    label: 'Target mode'
    inspector:
      group: 'document'
      editor: 'Neos.Neos/Inspector/Editors/SelectBoxEditor'
      editorOptions:
        values:
          firstChildNode:
            label: 'First child node'
            icon: 'icon-legal'
          parentNode:
            label: 'Parent node'
            icon: 'icon-fire'
          selectedTarget:
            label: 'Selected target'

Edit: Probably same issue >>> https://github.com/neos/neos-ui/issues/2042

Expected to be an array.

Change:
type: string

to:
type: array

in your “targetMode” property.