Very strange indeed. If the servo doesn't move when activating TH then the engine rpm shouldn't change. Have you tried to start it in normal mode? Lower your trim to pervent overspeed on start up. As stated before if you start in TH with very low carb arm position the motor can start backwards.
Did you check the starter direction, any problem with wires, battery or motor? Maybe under load the start gives up and the motor reverses! Hope you are not using an Align starter
Try another starter. If the start is ok you would hear the motor reverse as you are starting the motor. When does the motor actually reverse?
Check the direction of the check valve.
Please stand by for faster service!