We are aware of this...
It's a bug that doesn't update your driver's level as the experience increases.
So what usually happens in training sessions is that at some point you have all the attributes at 20 except experience.
So very often your driver is at a total level of 18 or 19.
If the experience increases further after that, it will no longer trigger an update of your driver's level.
One solution to this problem is to stop training, leaving an attribute of your driver at 19.
Then wait until your driver's experience is at 20 and then train the last skill from 19 to 20.
At that moment your driver will also jump to level 20. But this is only an aesthetic problem, the general level of your driver has no influence on his speed.
I have documented the issue again for the developers to investigate the cause, however, please note that they will address it depending on the priority of the tasks they currently have.