i had something simular but nowhere near as bad.
tho do drivers loose their training when they reach 20? had 1 of my drivers on 20 speed all of it then today it goes backwards?
There is no inconsistency in this drivers stats. I checked the history of his training and the progress and decline has been identical every time (even the day you reported this).
Did the driver perhaps go up a level? I know you said it went down, but the only way the blue bar should go from very high to very low is when levelling up. Other than that perhaps it was just a display issue.