Aha! It sounds like it is detecting the Up key or W key being held down. The character cannot move up and left/right at the same time, so left/right movement is being blocked.
Can you think of any reason on your end why it would detect one of these keys as being held?
It's taking me longer than expected to upload a new version because I found an unrelated bug, but if it's caused by the gamepad code, I'll have a new version with that code removed pretty soon.