A page automatically restore a older version?

Hi everyone!

I have a client site to maintain, which confused me is that when I update the home page, it will restore back to an older version after a while (don’t know how long it takes)for no reason. I’ve Changed the page 5-6 times and the page restored it to the unchanged version again and again.

What I’ve done:

1. Installed a Anti-virus/Malware plugin and scanned the file to see if the site has been hacked.
Wordfence Security << said that some core files had been altered, thus I restore them to the original one. But the page automatically restore problem remains.

2. Check the revision to see who make the changes, the weird part is that it showed that the changes was made by my account which I didn’t. I created a new account to change the pages but same scenario occurred.

The plugin and theme the site is using:

All-in-One WP Migration
Autoptimize
Contact Form 7
Contact Form 7 MailChimp Extension
Contact Form Submissions
Duplicate Post
Enable Media Replace
EWWW Image Optimizer
External Links
Fusion Builder
Fusion Core
Google Analytics for WordPress by MonsterInsights
Google XML Sitemaps
Instagram Gallery
Jetpack by WordPress.com
MINDBODY Widget
Popup Builder Platinum
Pretty Links
Smush
Select Ultimate Nofollow
Ultimate Nofollow
Select UpdraftPlus – Backup/Restore
UpdraftPlus – Backup/Restore
Wordfence Security
WP Fastest Cache
WP-Optimize
WPML Multilingual CMS
WPML String Translation
WPML Translation Management
Yoast SEO

theme:

Avada

PS: The page with problem is a translated page (the site contains 2 languages, used WPML to make translated pages.)

Any one may have ideas what is going wrong?

Thanks!