Our architect is currently planning the entire upper floor with drywall.
At first, I thought of sloping roof connections, but if I remember correctly, your upper floor has no sloping roofs. But many walls there do not stand above walls of the ground floor – so maybe there are structural reasons?
I see an advantage in drywall possibly in savings through own work.
The planner may have thought along similar lines, because ...
How high are the additional costs of a solid wall?
... one must not only think of material: properly masonry interior walls is, especially with frequently used smaller stone formats, a more delicate matter. It takes time, and that time of experienced workers. Drywall walls are, as you already notice from your own thought "own work," much more suitable for the use of semi-skilled helpers. The price of the stones is a secondary issue here, although aerated concrete blocks (which save integrating interior walls into exterior walls) are usually quite expensive in comparison. Further points have already been made by previous speakers.
By the way, "masonry" does not have to mean everywhere walls one and a half bricks (17.5 cm) thick; non-load-bearing walls of one brick (11.5 cm) thickness are also common.