Please, check now. Your custom message shows correctly in offline mode. There was a issues on offline.php, I replaced with this.

<?php
/**
 * @package Helix_Ultimate_Framework
 * @author JoomShaper <[email protected]>
 * @copyright Copyright (c) 2010 - 2021 JoomShaper
 * @license www.gnu.org/licenses/gpl-2.0.html GNU/GPLv2 or Later
 */

defined ('_JEXEC') or die('Restricted access');

use Joomla\CMS\Factory;
use Joomla\CMS\Helper\AuthenticationHelper;
use Joomla\CMS\HTML\HTMLHelper;
use Joomla\CMS\Language\Text;
use Joomla\CMS\Layout\LayoutHelper;
use Joomla\CMS\Router\Route;
use Joomla\CMS\Uri\Uri;

$template = HelixUltimate\Framework\Platform\Helper::loadTemplateData();
$this->params = $template->params;

$twofactormethods   = [];
if(version_compare(JVERSION, '4.2.0', '<'))
{
    $twofactormethods   = AuthenticationHelper::getTwoFactorMethods();
}
$doc                = Factory::getDocument();
$app                = Factory::getApplication();

ob_start();
?>  
<div class="coming-soon-login">
    <form action="<?php echo Route::_('index.php', true); ?>" method="post" id="form-login" class="mt-5">
        <div class="row gx-3 align-items-center">
            <div class="col-auto">
                <label class="visually-hidden" for="username"><?php echo Text::_('JGLOBAL_USERNAME'); ?></label>
                <div class="input-group mb-2">
                    <div class="input-group-text"><span class="fas fa-user" aria-hidden="true"></span></div>
                    <input name="username" type="text" class="form-control" id="username" placeholder="<?php echo Text::_('JGLOBAL_USERNAME'); ?>">
                </div>
            </div>

            <div class="col-auto">
                <label class="visually-hidden" for="password"><?php echo Text::_('JGLOBAL_PASSWORD'); ?></label>
                <div class="input-group mb-2">
                    <div class="input-group-text"><span class="fas fa-key" aria-hidden="true"></span></div>
                    <input name="password" type="password" class="form-control" id="password" placeholder="<?php echo Text::_('JGLOBAL_PASSWORD'); ?>">
                </div>
            </div>

            <?php if (count($twofactormethods) > 1) : ?>
                <div class="col-auto">
                    <label class="visually-hidden" for="secretkey"><?php echo Text::_('JGLOBAL_SECRETKEY'); ?></label>
                    <div class="input-group mb-2">
                        <div class="input-group-text"><span class="fas fa-user-secret" aria-hidden="true"></span></div>
                        <input name="secretkey" type="text" class="form-control" id="secretkey" placeholder="<?php echo Text::_('JGLOBAL_SECRETKEY'); ?>">
                    </div>
                </div>
            <?php endif; ?>

            <div class="col-auto">
                <input type="submit" name="Submit" class="btn btn-success mb-2 login" value="<?php echo Text::_('JLOGIN'); ?>" />
                <input type="hidden" name="option" value="com_users" />
                <input type="hidden" name="task" value="user.login" />
                <input type="hidden" name="return" value="<?php echo base64_encode(Uri::base()); ?>" />
                <?php echo HTMLHelper::_('form.token'); ?>
            </div>

        </div>
    </form>
</div>  
<?php
$login_form = ob_get_clean();
echo LayoutHelper::render('comingsoon', array(
    'language' => $this->language,
    'direction' => $this->direction,
    'params' => $this->params,
    'login' => true,
    'login_form' => $login_form,
    'isOffline' => true,
    'site_title' => $app->get('sitename')
));