You are oversampled. The default value is a midi value. But when oversampled, there can be more than one tick that represents this value. Despite what the display shows, there is no difference between a display of 0 and 1, they are both sending midi value 64.
The fix is to decrease your number of ticks so it matches the number of midi values, 128 in this case. You will also need to have the negative side go one more value than the positive to have 64 land on 0 - i.e., -99 to +98. If you want to stay at -99 to +99, as Dan recommends above, you could use named ticks to force midi 64 to be 0 displayed.