aframe-keyboard | A virtual reality keyboard and text editor in Aframe | Augmented Reality library

This is not a bug, but a slight misunderstanding of the keydown event on your part... The keydown event behavior can best be described as what you would expect to see when typing in a word processor, or notepad. if you hold down the I key you would see an I typed and then after a small delay, rapid repetition of I. Then additionally holding down J during that would stop the Is from being typed altogether, type a J, then after another small delay, rapid repetition of J.

What you should do instead, is once a single keydown event is being fired, set an internal variable to true, maybe call it something like throttle, steeringLeft, etc... then when a keyup event is fired for that key, set it back to false. have the car maneuver based on the true/false states of those variables.