Plasma 6.2.3 / Frameworks 6.7.0 / Qt 6.8.0 on 6.11.8-1 (Tumbleweed Slowroll).
On a desktop with two screens that show up fine on ddcutil detect and are fully controllable via the vdu_controls GUI tool.
The Brightness and Color widget does not show the screen brightness controls when I log in. Only if I remove the widget and then add it again, do the screen controls show up in the widget and then their brightness can be adjusted.
So, +1 from me on this.