Repro

• clone https://github.com/cedvdb/flutter_repros/tree/dropdown_selection_keys
• use branch dropdown_selection_keys
• flutter test

Result

The finder "2 widgets with key [<'option-hello'>] (ignoring offstage widgets)

code example

DropdownButtonFormField<String>(
       key: const ValueKey('dropdown'),
       items: const [
         DropdownMenuItem(
           key: ValueKey('option-hello'),
           child: Text('hello'),
           value: 'hello',
         ),
       ]