Widget block editor: Unable to collect checkbox values inside widget update() method

Question

Users can select an item from the list (the items are actually listed inside a DIV container). Upon selection, the item is added below with a leading checkbox in checked mode. When saved, the items get saved.

This flow works fine when I activate the Classic Widget editor.

But with the Block Widget editor, the checkbox values are simply getting ignored. I am not getting them inside $new_instance.

Here is the code written inside the update() method of the Widget:

function update( $new_instance, $old_instance ) {
   $instance = $new_instance
   ....
   ....
   // Unfortunately, these don't come through $new_instance
   $instance['groups'] = is_array($_POST['groups']) ? array_values($_POST['groups']) : array();
}

I am never getting any value inside $instance['groups'].

I added an additional textbox (<input type="hidden" name="instance-selected-groups" class="instance-selected-groups" value="' . $instance['instance-selected-groups'] . '" />) where I am storing comma separated item values using JavaScript like this.

I tried to grab these values in the update method like this:

if( empty( $instance['groups'] ) ) {
    if( strlen( trim( $_POST['instance-selected-groups'] ) ) > 0 ) {
        $instance['groups'] = array_pop( explode( ',', $_POST['instance-selected-groups'] ) );
        var_dump( $instance['groups'] );
    }
}

This doesn't work either.