I had this problem in my old MKV GTI after I had one front tyre replaced (following pot hole damage). I only replaced one tyre because they had only a few thousand miles on them and the opposite tyre looked perfect! However, the very slight difference in tread depth between the "old" tyre and the new one caused the car to pull sideways on acceleration.
I swapped the wheels from front to back and that solved it. Might be worth a try, swapping the wheels.
As others have said, any car will drift slightly to the left if you let go of the wheel. But it should be very very slight, you shouldn't have to consciously hold the wheel straight the whole time.