not flatten select

Hi!

I'm facing an issue where a select is not saving data when the options-array is not flat. I even tried using your exact demo-data, but it's not saved.
The selected value does make it into the POST headers, so it looks like some sort of validation that stops the data from being saved? The plugin even overwrites manually entered data, so it seems to be convinced that the field should be empty.
Changing 'flatten' to true/false does not seem to help.

'fields' => [
                    [
                        'name' => 'Beroep',
                        'id' => 'profession',
                        'type' => 'select',
                        'flatten'=>false,
                        'options' => array(
                          array( 'value' => 'monkeys', 'label' => 'Monkeys' ),
                          array( 'value' => 'king_kong', 'label' => 'King Kong', 'parent' => 'monkeys' ),
                          array( 'value' => 'curious_george', 'label' => 'Curious George', 'parent' => 'monkeys' ),
                          array( 'value' => 'donkeys', 'label' => 'Donkeys' ),
                          array( 'value' => 'eeyore', 'label' => 'Eeyore', 'parent' => 'donkeys' ),
                          array( 'value' => 'guss', 'label' => 'Gus', 'parent' => 'donkeys' ),
                         ),
                    ],