update_option_{$option} not working (do function after options are saved)

Question

Below is my code for a simple plugin settings page. It works and saves an option ("EN", "CZ").

However, what I intend to do is run a function after the options get saved (see at the very bottom). I use this wordpress function for it:

do_action( "update_option_{$option}", mixed $old_value, mixed $value, string $option )

Funnily enough, the very same code did work last week; right now, anything I try to echo inside the function just doesn't get printed, so I assume the function doesn't even start running.

Could anyone please point me in the right direction here?

<?php
add_action( 'admin_menu', 'cmi_csv_import_plugin_page' );

// tick
function cmi_csv_import_plugin_page() {
 
    add_options_page(
        'CSV Import - PUDR', // page <title>Title</title>
        'CSV Import', // menu link text
        'manage_options', // capability to access the page
        'cmi-csv-import', // page URL slug
        'cmi_csv_page_content', // callback function /w content
        5 // priority
    );
 
}

function cmi_csv_page_content () {
    ?>
    <div class="wrap">
        <h2>Import CSV</h2>
        <form action="options.php" method="post">
        <?php
        // this is what the setting is called and how you retrieve the option later! (with get_options)
        settings_fields( 'cmi_lang' );
        // slug name of the page whose settings sections you want to output
        // Use this in a settings page callback function to output all the 
        // sections and fields that were added to that $page with add_settings_section() and add_settings_field()
        do_settings_sections( 'cmi-csv-import' );
        submit_button( 'Aktualizovat metadata', 'primary' );
        ?>
        </form>
    </div>

<?php
}

// add an action on admin init
//
//
//
add_action ('admin_init', 'cmi_csv_admin_init');

function cmi_csv_admin_init () {
    // a/ register settings
    $args = array(
        'type' => 'string',
        // ↓ callable
        'sanitize_callback' => 'cmi_csv_validate_options',
        'default' => NULL
        );
    register_setting ('cmi_lang', 'cmi_lang', $args);

    // b/ add a settings section
    add_settings_section (
        'cmi-csv-section-main',
        'Import CSV (metadata PUDR)',
        // callable ↓ - echoes anything in the section
        'cmi_csv_section_text',
        // page name
        'cmi-csv-import'
    );

    // c/ add a settings field
    add_settings_field (
        'cmi_lang',
        'Jazyk aktualizovaného pole',
        // ↓ callback
        'cmi_csv_setting_jazyk',
        'cmi-csv-import',
        'cmi-csv-section-main'
    );
};

function cmi_csv_section_text() {

    echo '<p>Nastavte jazyk.</p>';

};

function cmi_csv_setting_jazyk () {
    // Get option 'beast_mode' value from the database
    // Set to 'disabled' as a default if the option does not exist
    $options = get_option( 'cmi_lang', 'EN');
    // Define the radio button options
    $items = array( 'EN', 'CZ');
    /*$jazyk = get_option ('cmi_lang');*/
    foreach( $items as $item ) {
        // Loop the two radio button options and select if set in the option value
        echo "<label><input " . checked( $item, $options, false) . " 
        value='" . esc_attr( $item ) . "' name='cmi_lang' 
        type='radio'/> " . esc_html( $item ) . "</label><br/>";
    }
}

function cmi_csv_validate_options ($input) {
    $input = sanitize_text_field( $input );
    return $input;
};
  
add_action('update_option_cmi_lang', function($old, $new) {

    if (!$new) {
        echo "didn't run";
        return;
    };

/// DO SOME STUFF 

}, 1, 2);

Answer 1

Answer: the code above works perfectly fine; it just doesn't echo anything inside the admin section. The 'stuff' I did with the update_option hook didn't work for reasons independent of the code above.