Steps to reproduce
1. Create a sprint with 2 stories with a few subtasks in each story. Set estimates on all subtasks. Start the sprint.
2. Check the burndown chart to verify that the number of hours remaining is correct.
3. Create a story in the backlog or a sprint that has not been started yet.
4. Move one of the subtasks from the current sprint from one of the stories. Choose change parent and select the story you created in the not started sprint / backlog.
5. The subtask is moved out of the current sprint / story, but the remaining hours in the burndown report is not updated with the hours moved out.
As a workaround I must always set remaining time for the subtask to 0 before moving, and then increase it again after it has been moved. If I forget this I am always confused and have to use more time than necessary on my reporting.