Kevin Bissell medal 5000 Super Mod 4 years 72 days ago
If you train a T20 driver to maximum in every attribute before experience gets to 20 the driver will usually top out at L18.
The way to make a driver get to L20 is to train all attributes except one (most people will leave one of the Mental attributes at 19) and then stop training. When the Experience attribute eventually reaches 20 (after many races / QRs) you resume training and when the final trainable attribute clicks over to 20 the driver level will also increase to 20.
According to the devs this is just a visual bug, it makes no difference to performance which is why they haven't wasted time fixing it.