Apparently the Mediawiki system treats multiple spaces in pagenames as equivalent to a single space. For instance if you go to
you get redirected to the proper page. And if you type
[[Category: West__Virginia]] in wikitext, it works as intended.
Question is to what extent this is followed, or should be, by external systems.
I haven't investigated WikiTree+. But on saving pages, I now get red banners from the new category error trapper saying "[[Category:West__Virginia]] does not exist".
So if you edit a page with no red links, and get an unexpected category error banner, although you haven't touched any categories, there's probably a double space in an existing category.