Dart FileUpdater works only on one field

Question

I have this piece of code that should update a .plist file

Future<String> getJsonAsString() async{
  var config = File('my_file.json');
  var stringContents = await config.readAsString();
  return stringContents;
}

Future<void> updateFields() async {
  var data = jsonDecode(await getJsonAsString());
  FileUpdater.updateFile(
    File('file.plist'),
    Plist(
      'field1',
      data['field1'],
    ),
  );
  FileUpdater.updateFile(
    File('file.plist'),
    Plist(
      'field2',
      data['field2'],
    ),
  );
}

my_file.json is a very simple json file with no nested data, like this:

{"field1": "value1", "field2": "value2"}

and file.plist is structured like this:

...
        <key>field1</key>
        <string>some_value1</string>
        <key>field2</key>
        <string>some_value2</string>
...

When I run my code, what I obtain is that only some_value2 is changed and becomes value2, while filed1 still have some_value1, like this:

...
        <key>field1</key>
        <string>some_value1</string>
        <key>field2</key>
        <string>value2</string>
...

If I delete the second FileUpdater, field1 is correctly updated. Is there something wrong in my code that I can't see?