Unable to customize filter/ search form of ModelAdmin in SilverStripe 4.4.4

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I am working on a SilverStripe project. Basically, I updated my project to SilverStripe version 4.4.4. After the upgrade, I found out that the search/ filter forms of the ModelAdmin were changed as in the screenshot below.

What I am trying to do now is that I am trying to customize the fields of the search/ filter form of the ModelAdmin following this lesson. https://www.silverstripe.org/learn/lessons/v4/introduction-to-modeladmin-1.

I have a data object class that is linked to a model admin class. Following is the dummy code of the model admin class.

class EnquirySubmission extends DataObject
    private static $db = [
        //some hidden fields are here

    private static $has_one = [
        'Member' => Member::class

    private static $summary_fields = [
        'Member.Name'   => 'Member',
        //some hidden fields are here

    //some hidden code goes here

    public function searchableFields()
        return [
            'Member.Name' => [
                'filter' => 'PartialMatchFilter',
                'title' => 'Member',
                'field' => \SilverStripe\Forms\DropdownField::create('Member.Name')
                    )->setEmptyString('-- Member --')

As you can see in the code, I am customizing the filter/ search form by overriding the searchableFields method. But it does not work in the upgraded version of SilverStripe. What am I missing and how can I fix it?