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When the shutter button is fully released, the camera stops the AF function, and the lens AF group remains on the last focusing distance just before the camera terminated AF.
However, if you continue to observe your target in the VF after letting go off the shutter button, you might see defocus develop because either your target and/or you have moved.
That is the way it is supposed to work. If you see anything different, then your gear may be in need of service.