How to update selective options on plugin settings page

I write a wordpress plugin. My plugin has options. All options are in one group, ‘pluginname-settings’. All option names are uniform and look like “pluginname-settings[‘option-name’]”. Some code to illustrate my case:

register_setting( 'pluginname_options', 'pluginname-settings', 'pluginname_validate' );
add_settings_section( 'section-one', 'Name of Section One', 'section_one_callback', 'pluginname_options-sectionname' );
add_settings_field( 'optionname', 'First Option', 'my_text_input', 'pluginname_options-sectionname', 'section-button', array(
    'name' => 'pluginname[optionname]',
    'value' => '42')
    )

For blog admins to tweak them, I created a settings page and grouped options in tabs. Each tab shows a subset of options (one section) and an “Update button”, as follows:

echo "<form action="options.php" method="POST">";
settings_fields( 'pluginname_options' );
do_settings_sections( 'pluginname_options-sectionname' ); 
submit_button('Update', 'primary',  'submit-form', false);
echo "</form>";

The problem is that when I press “Update” on one tab, the options from other tabs get cleared.

To my understanding, this is because all options represent an array of one setting.

Question: is it possible to save one section without clearing others under this setup? What is a good practice in such cases — register separate settings, use validation function to fill in the blanks or something else? Or is it just me doing it all wrong?

Thank you in advance.

P.S.: Per request in comments, here is a minimal working example:

<?php
/*
Plugin Name: Plugin Name
Plugin URI:
Description: Description
Version: 0.0.1
Author: Author Name
Author URI:
License: GPL3+
*/

add_action( 'admin_menu', 'pluginname_menu' );

function pluginname_menu()
    {
    add_options_page( 'PluginName Options', 'pluginname', 'manage_options', 'pluginname_options_page', 'pluginname_options_page' );
    }

add_action( 'admin_init', 'pluginname_admin_init' );

function pluginname_admin_init()
    {
    register_setting( 'pluginname_options', 'pluginname-settings' );
    add_settings_section( 'section-one', 'Section One', 'pluginname_section_callback', 'pluginname_options-section-one' );
    $options_array = get_option('pluginname-settings');
    add_settings_field( 'optionname-one', 'First Option', 'pluginname_field_callback', 'pluginname_options-section-one', 'section-one', array(
    'name' => 'pluginname-settings[optionname-one]',
    'value' => isset($options_array['optionname-one']) ? $options_array['optionname-one'] : '42')
    );
    add_settings_field( 'optionname-two', 'Second Option', 'pluginname_field_callback', 'pluginname_options-section-one', 'section-one', array(
    'name' => 'pluginname-settings[optionname-two]',
    'value' => isset($options_array['optionname-two']) ? $options_array['optionname-two'] : '42')
    );
    add_settings_section( 'section-two', 'Section Two', 'pluginname_section_callback', 'pluginname_options-section-two' );
    add_settings_field( 'optionname-three', 'Third Option', 'pluginname_field_callback', 'pluginname_options-section-two', 'section-two', array(
    'name' => 'pluginname-settings[optionname-three]',
    'value' => isset($options_array['optionname-three']) ? $options_array['optionname-three'] : '42')
    );
    add_settings_field( 'optionname-four', 'Fourth Option', 'pluginname_field_callback', 'pluginname_options-section-two', 'section-two', array(
    'name' => 'pluginname-settings[optionname-four]',
    'value' => isset($options_array['optionname-four']) ? $options_array['optionname-four'] : '42')
    );
    }

function pluginname_validate($input)
    {
    return $input;
    }

function pluginname_section_callback()
    {
    echo 'Feel free to change parameters below.';
    }

function pluginname_field_callback( $args )
    {
    $name = esc_attr( $args['name'] );
    $value = esc_attr( $args['value'] );
    echo "<input type='text' name='$name' value='$value' /> ";
    }

function pluginname_tab1()
    {
    echo "<form action="options.php" method="POST">";
    settings_fields( 'pluginname_options' );
    do_settings_sections( 'pluginname_options-section-one' ); 
    submit_button('Update', 'primary',  'submit-form', false);
    echo "</form>";
    }

function pluginname_tab2()
    {
    echo "<form action="options.php" method="POST">";
    settings_fields( 'pluginname_options' );
    do_settings_sections( 'pluginname_options-section-two' ); 
    submit_button('Update', 'primary',  'submit-form', false);
    echo "</form>";
    }

function pluginname_options_page()
    {
    ?>
    <div class="wrap">
        <h2>Options</h2>
        <h2 class="nav-tab-wrapper">
        <a href="?page=pluginname_options_page&tab=1" class="nav-tab <? if ( @( $_GET['tab'] == '1' ) || !isset($_GET['tab'])) echo "nav-tab-active"; ?>">Section One Options</a>
        <a href="?page=pluginname_options_page&tab=2" class="nav-tab <? if ( $_GET['tab'] == '2' ) echo "nav-tab-active"; ?>">Section Two Options</a>
        </h2>
        <?php $current_tab = $_GET['tab'];
        switch ($current_tab)
            {
            case '2' : pluginname_tab2(); break;
            default: pluginname_tab1();
            } ?>
    </div>
    <?php
}
?>

minimal working example.