Change scheme of filefield path

Hello.

I have a custom content type - Media Library File.
It has a thumbnail field of image type.
I want thumbnails to be stored in private:// so I chose "private" in admin/structure/types/manage/medialibfile/fields/medialibfile_field_thumbnail.
Now it does save the thumbnails in private:// but when the image is displayed the path points to public:// so I get error 404 and broken image icon.

In the module that adds mentioned content type, in install file, in fields I have:

[
      'field_name' => 'medialibfile_field_thumbnail',
      'type'  => 'image',
      'settings' => [
        'uri_scheme' => 'private',
      ]

...and in the same file, in fields instances:

    [
      'field_name' => 'medialibfile_field_thumbnail',
      'entity_type' => 'node',
      'label' => 'Thumbnail',
      'bundle' => 'medialibfile',
      'description' => 'A thumbnail attached to Media File of video type.',
      'settings' => [
        'file_directory' => 'medialibfile',
        'file_extensions' => 'bmp gif png jpg jpeg svg',
        'max_filesize' => '100MB',
        'title_field' => '',
        'filefield_paths_enabled' => 1,
        'filefield_paths' => [
          'file_path' => [
            'value' => 'medialibfile/[file:timestamp:custom:Y/m]',
            'options' => [
              'slashes' => 0,
              'pathauto' => FALSE,
              'transliterate' => 0,
            ],
          ],
          'redirect' => FALSE,
          'retroactive_update' => 0,
          'active_updating' => 0,
          'file_name' => [
            'value' => '[file:ffp-name-only-original].[file:ffp-extension-original]',
            'options' => [
              'slashes' => 1,
              'pathauto' => FALSE,
              'transliterate' => 0,
            ],
          ],
        ],
      ],

This line:
'value' => 'medialibfile/[file:timestamp:custom:Y/m]',
...is the path but it points to public://medialibfile/... and I want to change it to private://.
I tried to change the path during submission of the Medialibfile form but all paths point to public://filefield_paths which is a temporary directory.

Any ideas why is there public:// in the path despite files being saved in private://?