Why does If-Modified-Since not work with Litespeed WordPress plugin and response code is always 200?

Question

LiteSpeed ​​Web Server

Hello! The LiteSpeed ​​plugin is installed on WordPess, the hosting also works on LiteSpeed.
Added code:

<?php
$LastModified_unix = 1594844676;
$LastModified = gmdate("D, d M Y H:i:s \G\M\T", $LastModified_unix);
$IfModifiedSince = gmdate("D, d M Y H:i:s \G\M\T", $LastModified_unix);
if (isset($_ENV['HTTP_IF_MODIFIED_SINCE']))
    $IfModifiedSince = strtotime(substr($_ENV['HTTP_IF_MODIFIED_SINCE'], 5)); 
if (isset($_SERVER['HTTP_IF_MODIFIED_SINCE']))
    $IfModifiedSince = strtotime(substr($_SERVER['HTTP_IF_MODIFIED_SINCE'], 5));
if ($IfModifiedSince && $IfModifiedSince >= $LastModified_unix) {
    header($_SERVER['SERVER_PROTOCOL'] . ' 304 Not Modified');
    exit;
}
header('Last-Modified: '. $LastModified);
header('If-Modified-Since: '. $IfModifiedSince);

?>

The problem is that the If-Modified-Since header does not work and the server response code for pages is always 200, not 304. If the plugin is disabled, then If-Modified-Since is displayed normally and the response code is 304.

I also tried the Cache Enable plugin from Key CDN, it automatically adds the “Last-Modified” and “If-Modified-Since” headers, but for some reason this did not work on this hosting either.

Can you please tell me why when the LiteSpeed ​​plugin is enabled, the "If-Modified-Since" header does not work and the server response code is always 200?