Hey @core365,
The disappearing content issue after the update could happen due to multiple factors, you could check for the following:
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Ensure everything is up to date including WordPress according to our version compatibility list at https://theme.co/apex/forum/t/troubleshooting-version-compatibility/195. Please follow the best practices when updating your theme and plugins. See https://theme.co/apex/forum/t/setup-updating-your-themes-and-plugins/62 for more details.
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If you’re using a caching plugin, clear all caches including browser cache then deactivate your caching plugin and other optimization plugins. If you’re using a CDN, please clear the CDN’s cache and disable optimization services.
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Test for a plugin conflict. You can do this by deactivating all third party plugins, and seeing if the problem remains. If it’s fixed, you’ll know a plugin caused the problem, and you can narrow down which one by reactivating them one at a time.
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Remove custom CSS, JavaScript and deactivate your child theme and switch to parent theme (take a complete backup first).
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Make sure the memory limit is at-least 512M, if not then you can increase the allocated memory by adding this code in your wp-config.php (https://theme.co/apex/forum/t/troubleshooting-increasing-php-memory-limit/16935)
define( 'WP_MEMORY_LIMIT', '256M' );
define( 'WP_MAX_MEMORY_LIMIT', '512M' );
For updating the site I would recommend that you create a staging site. This is very useful because you can test drive any X, Cornerstone or Pro updates in the staging area, do the customization and troubleshooting without disrupting your live site. Once everything is perfect, you can easily and flawlessly proceed the update in the live site. To know how to create a staging area, please check out these articles:
http://www.wpbeginner.com/wp-tutorials/how-to-create-staging-environment-for-a-wordpress-site/, https://maintainn.com/2015/02/how-to-create-a-staging-site-for-wordpress/.
After doing the updates, always remember to clear all caches (if you are using WP Rocket, WP SuperCache or W3 Total Cache) when updating so that the code from the latest release is always in use. This will help you to avoid any potential errors.
Hope this helps!