We recently updated from 1.9.something to 126.96.36.199, and are experiencing the same problem. It manifests in two slightly different ways, and is most visible when dealing with tables:
A) This post has a table that was built in Dreamweaver and copy+pasted into our forum.
When I click Edit, the post appears normally in the editor.
When I preview or save, it displays raw HTML code to all users.
B) This post has a table that was built in TinyMCE HTML's table tool.
When I click Edit, the editor window is completely blank.
When I preview or save, it displays raw HTML code just like the first example.
This problem happens consistently on multiple computers since we upgraded, in all three primary browsers (IE/Firefox/Chrome), and in both TinyMCE HTML and CKEditor HTML. However, one of the forum moderators doesn't have the same problem-- if I edit one of these posts and save, it shows as HTML code. If he then edits it and saves without changing anything, it shows up as formatted content. His system information is the same as mine-- a Windows 7 PC running Firefox 29.0.1 and using CKEditor HTML, so I have no idea why it works for him and not for anyone else.
Please help! 😞