Dealing with an employee that went over my head

I manage a small team and recently hired for a new position. In the job posting it clearly stated that the work hours are from 8:00-4:30. No concerns about these hours were mentioned during the interview process at all.

Recently after accepting the job offer, the new hire expressed an issue with the hours of work, and proposed their own hours. The hours were not even consistent every day, they requested different work hours for each day of the week (they did all add up to the same total number of hours). I rejected this request.

The employee(before the first day of work), then emailed my superior with the same request and was approved.

How do I carry on managing an employee that is willing to go right over my head on an issue like this? How do I address this with my manager so that it doesn’t happen again. ( the first words out of my managers mouth should have been “Have you discussed this with your direct manager first?” )