To reproduce:
1) Open attached project and scene
2) Play it
3) Left-click somewhere to move there
4) While midway, move with WASD keys
Expected: WASD works, allowing to temporarily move elsewhere before returning towards destination
5) Let it finish its path to destination
6) Try moving with WASD keys
Actual: WASD no longer works

Workarounds:
a) Call ResetPath(). There are two different modes in playerController 'Reset Mode' drop-down choice for that.
"Only if Has Path" will call ResetPath() only when the player is midway and you try to move with WASD - if the destination is not finished and path is reset, you can move with WASD freely. If the player finishes moving to the destination without input, WASD won't work.
"Always" will call ResetPath() always, meaning that even after stopping in its left click destination the player will be able to move
b) Turn off Auto Braking and increase Stopping Distance to at least 0.5 (because lesser values cause semi- to fully permanent wobbling). Even when the player stops by its destination will it be able to move.

Reproduced in 2017.2.0f3, 2017.3.0f3, 2018.1.0b7