Syntax Conversion Fails Without Error (and Messes up Page)

Experienced with latest LTS release:

1. Changing the Syntax in WYSIWYG edit mode (tested with changing from XWiki 2.1 to  Markdown 1.2) asks if the page syntax shall be converted.
2. After confirmation, the page is still displayed in the WYSIWYG editor just fine.
3. When saving, the page looks all messed up - reason is that the page syntax was NOT actually converted, contrary to the information from the popup.
4. Changing the syntax back to XWiki 2.1 with additional syntax transformation then really messes up the page this time, as then it's actually converted, but (logically) interpreting the XWiki 2.1 as Markdown 1.2, and escaping everything...

You can see a screen recording of steps 1-3 in the attachment.