Help:Categories#Linking to a category shows the need for a leading colon when linking to a category-page:
However, in MW 1.31, i'm creating the link using the WikiEditor toolbar-- the syntax should be correct, but it's not. Therefor, this appears to be a bug with the WikiEditor toolbar. It fails to include the leading colon when linking to a category.
I test with verbose error-reporting, and didn't see any errors.
Reported 3 years ago, still broken. https://phabricator.wikimedia.org/T38227
Feel free to mark as "duplicate".