This is an answer to an issue I recently faced.
Background
I recently migrated my Wordpress site from my local machine to a server at siteground.com. I used the plugin duplicator to do the migration. Every time I tried the migration the website would appear properly on the front end (with the exception of a lot of question marks in diamonds replacing certain charachters.) In the backend I could see all of the pages and click in to edit them. Once on a particular pages admin edit screen no content would appear! I could click revisions and see the revision history but nothing would appear in the actual content box. Pressing update would clear that pages contents.
Potential Causes
Database errors. Plugin conflicts.
The Cause for me
In my wp-config the Database Charset to use in creating database tables was set to
define('DB_CHARSET', 'utf8mb4');
Apparently Wordpress is in the process of upgrading tables to utf8mb4. See here for reasoning, https://make.wordpress.org/core/2015/04/02/the-utf8mb4-upgrade/.
The issue is my server at siteground had it's tables set to utf8. And my local environment had a new wordpress install with the upgrade to utf8mb4.
The Fix
A simple change from utf8mb4 to utf8 did it for me.