When an automatic engine is in Drive, you can drive forward like normal, and you won't be able to shift into gear since you're already in gear... until you get into anything higher than first gear.
After the car automatically shifts up, the driver can spam the ShiftUp control which momentarily puts the car into first gear, and then back into the automatically chosen gear!
This can't be good for engines, transmissions, or the shifter itself...

Fixed in V18.3.0.

Yup, that's a bug all right. But an odd one.