Hi Steven, the symptom you describe is centre-braking, a common feature in RC car speed controllers allowing the operator to brake/slow the car when moving forward before applying reverse. It's fine for RC cars because you generally need to brake but don't necessarily need or want to throw it into reverse straight away, less ideal for combat robotics since you need to be able to change from forwards to reverse almost instantaneously in quick succession.

Some speed controllers have a programming feature allowing you to disable centre-braking but, having had a look at the specs for that controller, I can't see anything mentioning that, so you may need to look at a different controller as it's not really possible to operate a robot with the accuracy needed while centre-braking is active.