Post changes don’t save in admin

Right so this is a weird one and so far I can’t find a solution or anyone else who has had the exact same issue:

Issue
When editing specific posts (happens seemingly randomly on just a few posts here and there), changes aren’t saved within the admin. Nothing to do with the theme; any changes saved to the post (e.g. permalink, content, etc) don’t persist after updating the post.

Already tried

• Disabling all plugins
• Removing Multisite functionality
• Repairing DB
• Restoring wp-config.php to default
• Hitting up WPEngine support (they have no idea)
• Checked error logs – no errors are outputted
• Removed all custom fields
• Manually made changes in DB (this is the only way changes persist)

Pretty much I’ve pared the install down to a clean install and yet this is still happening. Is there somehow a post can get corrupted and thus not save properly? I have no idea at this point.

Any help is greatly appreciated!