It may be caused by browser cache , you can clear the cache and see if the column headers can display normally . Or you can check your report in another browser to see if the column header can be displayed normally .
Community Support Team _ Ailsa Tao
If this post helps, then please consider Accept it as the solution to help the other members find it more quickly.
There was a fix recently for headers not working properly in the service. It should be in Prod by end of next week I beleive. If you still have the issue after the 15th, you might want to file a service request as what you say is a bug that should be fixed.