This is not a bug, it is a default behavior. The additional URLs saves the old URLs in order not to break the front end of the site. I have logged a feature request about the ability to delete the group page and all the additional URLs of the old child pages to be updated automatically. For your convenience you can follow the feature request status in PITS on this public URL
and vote for its popularity. Hopefully will be able to implement it for the next releases.
the Telerik team
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items