Did someone come across this one?
<inputbox> type=create prefix=Extra: preload=Template:PreloadWithCategory buttonlabel=Create new page useve=true </inputbox>
Template:PreloadWithCategory
[[Category:ExtraPage]]
This adds category "Index.php?title=Category:ExtraPage" instead of just "ExtraPage". In other words, it prepends "Index.php?title=Category:" to the intended category name.
I am unsure if this is caused by the default URL schema (https://example.org/index.php?title=PageName, or VisualEditor, or a combination of both. We are talking about MW 1.42 here.