First noticed by the following steps:

1/ Assign a user a role which has moodle/category:create allowed to a category
2/ As that user, try to add a subcategory from the category
3/ The form will be displayed to add a new category
4/ Submiting the form returns you back to the form

Looking at the top of categoryedit.php seems to indicate that when categories are being added with also use system context.. Need to dig further..