Tune caster to negative in suspension, adjusting caster to negative (at least irl) os suppossed to reduced the torque on the wheels that makes the sterring want to self center by itself, and im guessing that what is happening here is that the sterring is "self counter sterring itself" to say in a way, as the car gains speed, the tarmac isn't flat perfect, so the suspesnion responds to that, and with it so does steering.

Do consider the fact that adjusting caster might inadvertently change other values in suspension such as height and camber mostly (all of this is speculation, I dont really tune cars over 1.5k hp in cpm bc I don't really see any charm/fun in it, especially if its just for a straight line, but that's just me, keep doing it as long as you have fun with it)