This might be the cause:
https://discuss.kde.org/t/powerdevil-in-plasma-6-0-and-beyond/14548
Note this paragraph:
Brightness via DDC/CI will not be animated anymore in order to minimize the risk of shortened monitor lifespans. The chance of this happening is hard to verify for someone not in the monitor industry, but we’ll follow the example of monitor manufacturers’ own helper applets and apply newly adjusted monitor brightness values only after a half-second delay.
Lifespan in this case refers to the nvram write limits - which for some/old monitors may be quite low.