With speed reduction, the autopilot will command pitch changes to maintain altitude. These pitch changes (up/down) will be seen on the active sidestick.
In fact, you will naturally expect to see these pitch changes, because if you were to fly the plane manually you will make those same pitch adjustments.
In a conventional aircraft that would be correct. In an Airbus it isn't. Remember, the airbus is flight path stable, not speed stable. It will maintain the flight path without pilot input within the normal envelope.
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The same thought experiment in manual flight, manual thrust. Pull back thrust and maintain the stick centered (just don't touch it). The aircraft will slow down and maintain (roughly) the same altitude with the same pitch change to do that.
The FBW system, not the autopilot, not the human pilot, will command the necessary control surface deflections. And if you have seen the
"dance of the ailerons" after take off on an A380 for example you would know that any feedback of those control surface deflections would be very counter productive.