In the new update for Windows 10 on 8/11/15, when a player double taps "w" to sprint, it does the normal effect, however when the player stops moving they are still sprinting. This is fixed only when the player walks into a block.
Linked issues
Comments 4
Duplicate of MCPE-9807
Confirmed for the August 11 update of Windows 10 Edition v0.12.0.