On the track it won't feel twitchy since there are no bumps. But you still get bad handling because of the beam. Bump steer does not only mean the toe changes on bumps, it will change on any compression and rebound of the spring, so also on cornering. And the bad thing during cornering is that the inside wheel will toe out and the outside wheel will toe in, so both rear wheels are working against the front wheels when trying to rotate the car, and the result is understeer.

In an oversteer situation when you have opposite lock on and when the rear then suddenly grips, the rear wheels are then pointing in the opposite direction of the front wheels, which makes it very easy to loose control and spin the car. And the softer the springs and shocks are, the bigger the movements of the springs will be, and therefore the bigger the toe changes will be on the rear.