Issues with PHP Session Persistence Post-Server Migration

I've recently migrated  PHP web application to a new server and are encountering issues with session persistence. Users are experiencing frequent logouts and session data seems to vanish unexpectedly. Our setup uses PHP 7.4 on an Apache server.

1. I verified session.save_path is writable and accessible.
2. I checked cookie settings and confirmed headers are sent before outputs.
3. Ensured no session_id regeneration occurs unexpectedly.

Could this be related to specific server settings or PHP configurations that differ from our previous environment? 
 

Here the problematic code.

Any guidance!

<?php
// Start the session
session_start();

// Example of problematic session code
if (!isset($_SESSION['initialized'])) {
    // Attempting to initialize session variables
    $_SESSION['user_id'] = fetchUserId();
    $_SESSION['initialized'] = true;

    // Regenerating the session ID - can cause issues if done improperly
    session_regenerate_id();
}

// Check if the session has a specific variable
if (!isset($_SESSION['user_data'])) {
    // Mismanaging session data
    $_SESSION['user_data'] = loadUserData($_SESSION['user_id']);
}

// Incorrect use of session_write_close
session_write_close();

// More processing which might need session data
processUserData($_SESSION['user_data']);

function fetchUserId() {
    // Placeholder for user ID fetch logic
    return rand(1, 1000);
}

function loadUserData($userId) {
    // Placeholder for loading user data logic
    return "UserData for user ID: $userId";
}

function processUserData($data) {
    // Processing logic here
    echo "Processing $data";
}

?>