I have an old Acer V203W as part of my multi monitor setup. It sometimes just doesn't appear in the brightness controls for KDE. I haven't noticed any correlation between package updates, but maybe something is weird there I am missing? I disabled my GPU under-clock and swapped the cable to different ports on the GPU. I am not sure if this is a config issue somewhere on my part or if it is a KDE bug. I am not sure what kinda log's are need for anyone to lend any assistance so please let me know and i will add an edit with the needed info. My linux knowledge probably has some large gaps as most of the time stuff just works for my use case so please be patient :3
For what it's worth, it happens to me as well, but when it does, it occurs on both monitors at the same time. One a 4k 244Hz OLED monitor, the other an older QHD. I put it down to a Plasma bug