Break/reverse is listed in the in-game tooltip (up on d-pad while in a car) as [ACTION-BREAK] instead of an actual control.
It's clearly supposed to be LT, but it's not mapped correctly.

Even resetting the controller config to default doesn't fix it. This isn't user-side. The default controller setup doesn't have break/reverse mapped to a button.
What the ♥♥♥♥, Ubisoft?