The camera is meant to be moving freely if you don't activate physics, hence the name Freecamera. These are things that are normal and that were made by the BabylonJS devs.
A Freecamera goes forward when you press W and backwards if you press S, it takes under consideration the rotation of the player.
So if you wanna make your own settings, then I recommend you change to Targetcamera instead of Freecamera, that way the camera behavior won't interfere with your settings and controls.
On keypress W:
Set camera position to (Camera.FrontXPos(1),Camera.FrontYPos(1),Camera.FrontZPos(1))
On keypress S:
Set camera position to (Camera.FrontXPos(-1),Camera.FrontYPos(-1),Camera.FrontZPos(-1))