You should not be using that switch much. It is "set it once and walk away" item. The middle position turns the actuator off so that the automation doesn't make it move. Since there is no standard for which port is pool or spa, it just lets you set a default "pool" position and the automation will cause it to turn the other way for spa operation.
The actuator itself comes from the factory with what should be a 180-degree sweep from one side to the other, but it isn't always that way. Manufacturing errors do happen. Installers don't always check.
There is an internal cam that opens and closes tiny switches under the cover to set the stop position. These need to be set accurately so that the valve itself closes or opens correctly. Very easy on Hayward actuators. Yours may not be closing that side completely and allowing water to flow back from the spa when the pump is opened. It will also allow the pump to pull a small amount from the spa while in "pool" mode. That is not the best thing to have happening. You may have not even noticed as the return valve is set so the spa overflows, as is normal.