Hi - this is the first time I've heard of this problem.
The movement input works by listening for input on an axis, which has a deadzone of 50%. The only reason I can think of for this happening is that your keyboard's down key is sitting above that threshold at all times. Even if you are using a gamepad, the game still listens for keyboard inputs and switches if a key is pressed.
I know it isn't optimal, but you could try removing the keyboard from it's USB slot before/when you start the game, and then trying controller input. If the issue persists, either your gamepad is also above 50% (seems unlikely) or you've found a bug in the game I'll need to try and replicate. If it doesn't, then your keyboard down key must be registering as pressed.
Let me know if you try removing the keyboard and what the result is.