This was reported initially on MediaWiki.org. I have seen it on wikis with REL1_39 and REL1_42 with the respective extension codes using the default URL schema (https://example.org/index.php?title=PageName).

<inputbox>
type=create
prefix=Extra:
preload=Template:PreloadWithCategory
buttonlabel=Create new page
useve=true
</inputbox>

Content of Template:PreloadWithCategory:

<includeonly>
[[Category:ExtraPage]]
</includeonly>

This adds the 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), VisualEditor (useve=true), or a combination of both.

I am unable to replicate the issue on a wiki using short URLs.