This issue is addressed by the neutral valve. I am suggesting you take it to the dealer or get the service manual.
here is some dialog from the manual.
0°, the rotating speed of the output shaft in the hydraulic
motor should be 0 since there is no oil supply. However,
it is almost impossible for the driver to adjust the HST
swash plate to 0°. If the angle of the swash plate is out
of 0°, the tractor starts to move and keeping the neutral
position is very difficult. To solve this problem, the neutral
valve is installed to secure the neutral range.
The neutral valve (1) drains the oil in the circuit through
its orifice so that the hydraulic motor does not rotate
when the amount of oil supplied to the motor is insufficient
even though the angle of the swash plate in the
pump is close to 0° (not exactly 0°).
As the swash plate angle is increased, the pressure of
the oil supplied by the pump increases. This pressure
increases to the point that the pressure is greater than
that of the return spring pressure of the neutral valve.
The neutral valve moves to close the orifice.
There are adjustments on the Hydrostat pedal shaft linkage. And that could be the problem That is listed as a possible problem along with the neutral valve, it could be either. .. There are several diagrams. with measurements to adjusting and tension nut and shaft length.. You really need to study the manual if you are going to tackle this. Otherwise, load it up and take it to the dealer.