Thread:Extension talk:WYSIWYG/IE11 with MW 1.22.0 + WYSIWYG extension: Existing category definitions will corrupt after after page has been saved/reply (6)

Problem was caused by this IE8 problem, for some reason with IE8 there where extra attribute "data-cke-expando=" left in code when editing mode was switched to wikitext. I had not applied that fix in my installation earlier. Your modified test seems to work with IE 8 without problems as does my #6 installation too.