I recently got a ThinkPad T14 Gen 4 with AMD Ryzen 7 PRO 7840U w/ Radeon 780M Graphics. Brightness controls are working fine on Manjaro Gnome, however, once I lock the screen (display blanks) and then if I wake the device, the brightness is different from what was set previously.
The funny thing is that control file for brightness
/sys/class/backlight/amdgpu_bl1/brightness will report the same old brightness, but if I write the same value to file again, the brightness reduces significantly.
$ inxi -G
Device-1: AMD Phoenix1 driver: amdgpu v: kernel
Device-2: Syntek Integrated Camera driver: uvcvideo type: USB
Display: wayland server: X org v: 126.96.36.199 with: Xwayland v: 23.2.1
compositor: gnome-shell v: 44.5 driver: gpu: amdgpu
resolution: no compositor data resolution: 2880x1800
API: EGL v: 1.5 drivers: radeonsi,swrast
API: OpenGL v: 4.6 compat-v: 4.5 vendor: amd mesa v: 23.1.9-manjaro1.1
renderer: AMD Radeon Graphics (gfx1103_r1 LLVM 16.0.6 DRM 3.54
EDIT: Seems to be fixed in the latest 6.6 kernel.