Wikitext export option no longer available in LO Writer, even though the Mediawiki extension is installed. Why?

At the moment I’m using version for Microsoft Windows. I’ve previously used 5.0 but found the Mediawiki export option there added a whole bunch of unnecessary HTML junk to the export.

However, now I find the export option isn’t available at all in I’ve tried uninstalling and reinstalling it twice but it’s still not there. I’ve checked the extension manager and it reports the extension, which came with LibreOffice, is installed.

Why can’t I export as Wikitext anymore? I really need that export option.