We have had our Kona for almost a year now and have had this issue from time to time as well, it is not speed dependent but it is entirely based on the resetting the brakes as you have noticed. The only reference to this that we have found is in the regular cruise control section on page 5-89 of our owners manual:
During normal cruise control operation,
when the SET switch is activated
or reactivated after applying the
brakes, the cruise control will energize
after approximately 3 seconds.
This delay is normal.
• Before activating the cruise control
function, the system will check to
verify that the brake switch is operating
normally. Depress the brake
pedal at least once after pressing the
POWER button to the ON position
or starting the vehicle.
If we cant get the cruise to set we just ever so lightly tap the brake pedal and you're good to go, perhaps when leaving we don't depress the brake fully as the system needs a reset to get its head straight.
Also remember that one programmers flaw is another's 'undocumented feature'!