

I saw this problem for the first time yesterday. Run dmesg
to look for errors from the kernel, for me I had amdgpu 0000:03:00.0: [drm] *ERROR* dc_dmub_srv_log_diagnostic_data: DMCUB error - collecting diagnostic data
. I took this to mean that my system couldn’t communicate with the monitor to change brightness anymore. When my system is idle, it first dims the monitor before turning it off, so when I wake it back up it’s stuck on low brightness like this. Simply turning my monitor off and back on seemed to fix it.
Nice