Do you know what version of Discourse your site is using? If your site is self-hosted, you can find the version near the top of the site’s admin dashboard. The reason I am asking is because I think Discourse should have prevented you from adding the parent category. When I try to do it, I get the following error:
It is possible that you are running into a bug that has been fixed in a more recent version of Discourse. It is also possible that I am not reproducing the issue correctly.