Gutenberg Editor already maintains the parent-child relationship when you select a category. So the child doesn’t move at the top in the category meta box. So this plugin isn’t required if you’re using the Gutenberg Editor.
Categories in Hierarchical Order plugin applies to WordPress Classic Post Editor only.
This plugin contains functionality to let you maintain default hierarchical order of categories when you select and save them for a post in your WordPress Admin Classic Post Editor.
The selected category/categories, to which a post belongs, are by default pushed at the top in the Category tab while saving the post. Whereas activating Categories in Hierarchical Order plugin will keep the position of selected category/categories unchanged as per their parent/child relationship.
For support, use support page available here or visit the page at our official website. A GitHub repo is also there to post issues, contribution and feature requests.
Screenshots
WordPress default category order
Category hierarchy maintained by the plugin