I've just installed the extension and added the patch for WikiEditorHooks.php. The wiki is still new and doesn't have much content so on most pages with existing text, TinyMCE and rich text are showing up fine. On at least one page though, which has a Heading 2 text and some normal text, TinyMCE shows up but the text entry area is blank. I can enter text, but whatever I typed doesn't get added to the page.
Not sure if it's relevant, but I'm running MW 1.31.1 with SMW 3.0.0, on IIS 8.5 running on Windows Server 2012 R2. I'm accessing the wiki via Chrome 70 on macOS 10.14.1.