Upon sprinting while flying once, you stay sprinting until you land or stop flying, and you move faster in every direction, not just forwards. What used to and should happen is that you'd continue sprinting until you stopped flying forwards, and sprinting speeds wouldn't affect backwards and horizontal flying.

https://gfycat.com/MiniatureHandsomeKittiwake
(File was too large to attach, so I uploaded it to gfycat.)