Why aren’t sub-categories nested after creating post in the WP Admin?

I have several sub-categories set up. Everything displays properly as far as nesting (see below) when creating a new post.

Category 1

–Sub Category 1

–Sub Category 2

–Sub Category 3

Category 2

etc

After I select a sub-category, then go back and edit a post, the sub category that I check shows up like this (note how it’s not nested under Category 1):

(checkmark) Sub Category 2

Category 1

–Sub Category 1

–Sub Category 3

Category 2

etc

Is this a bug in WordPress 3.1.2? Can someone tell me how to fix it? I don’t remember it doing this when I was running 3.0.5 (but there’s a chance it could have).

This “feature” has been in WP for quite a while.

You can disable it by installing this small plugin:

http://wordpress.org/extend/plugins/category-checklist-tree/