This is because model is not moving at all. You will give some input, but model will do nothing.
If you are flying, then you will give input and model will move to the point where you commanded to. And since model is moving, it will return the swashplate just according movement.
So it will level the swashplate just once it will reach position or just rotation speed you want with stick input.

This is job of each flybarless unit. Without such reaction it would act as pure receiver or would be not that stable if reaction would be whatever else.
Since gyros are reacting very fast to the model movement, there is no delay in the air. But since on the ground there will be never proper movement it will always only try to achieve desired position only to a few seconds, until it will level on its own.

You are comparing it with receiver. But receiver task is to only send stick input directly to the servos. This is very different than what controllers are doing.
With flybar mechanics there are paddles that are doing similar work. But obviously you cant see their action, because rotor is not spinning. Its stability is limited by mechanical properties of the paddles. While with flybarless controllers you can make the virtual paddles simply ideal, not limited by any mechanical limitations.

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Spirit System developer

I understand and accept your answer.

Another strange anomaly if I release the cyclic stick and let transmitter stick spring return it to neutral it very slowly levels. If on other hand the stick is pushed to center it moves much more rapidly.

If you give the sticks even the slightest tweak over centre the unit will interpret that as a command in the opposite direction and follow your request for roll or pitch instantly. Springing the sticks to centre will do just that and the unit will not have seen a request for an opposite command. Bear in mind the sticks and servos are not directly connected as in a flybar setup. This is fly by wire.

In other words, by looking at the servos at the bench you can't find a real behavior in the air. It will behave very diferently when in the air, because there is motion feedback from the model.

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Spirit System developer