I found this same issue today with a client’s website; when creating a new post or editing an existing one, any text entered into the ‘Visual’ tab of the post editor would not appear in the ‘Text’ tab – nor would it appear when the post was previewed or published.
I went through and deactivating all the site’s plugins – and the issue was solved. So I went through each plugin reactivating them one-by-one until the problem resurfaced. (You could do this the other way around – deactivating plugins until the problem is fixed. Probably way less invasive. Oh well. #YOLO.)
The problem resurfaced when I reactivated the Yoast SEO plugin. Having not spoken to the client in depth yet I’m not sure when they last updated their installation of the plugin – but if you’re running this plugin too, try deactivating it and seeing if your problem is still there. This is odd, though – as the developers of the Yoast SEO plugin are usually quite ‘on it’ – and I run the plugin on several other sites which haven’t had the same issue.
*NB. I noticed that when the problem was current – the mouse cursor in the visual editor would start right at the top of the text field. This is odd behaviour, as usually there is a noticeable line-break between the bottom of the WYSIWYG editor and the first place you can actually enter text (even more noticeable when compared to the cursor position when in ‘Text’ tab view).
